The Biology of Aging

Part 5: Aging

Population aging is transforming the world in dramatic and fundamental ways. The age distributions of populations have changed and will continue to change radically, due to long-term declines in fertility rates and improvements in mortality rates (Table 93e-1). This transformation, known as the Demographic Transition, is also accompanied by an epidemiologic transition, in which noncommunicable chronic diseases are becoming the major causes of death and contributors to the burden of disease and disability. A concomitant of population aging is the change in key ratios expressing “dependency” of one form or another—the ratio of adults in the workforce to those typically out of the workforce, such as infants, children, retired “young old” (those still active but in ways other than paid work), and the oldest old. Global aging will affect economic growth, migration, patterns of work and retirement, family structures, pension and health systems, and even trade and the relative standing of nations. Both absolute numbers (the size of an age group) and ratios (the ratio of those in working ages to dependents such as the young or retired, or the ratio of children to older people) are important. The size of age groups might affect the number of hospital beds needed, whereas the ratio of children to older people could affect the relative demand for pediatricians and geriatricians.

Although the increase in life expectancy, resulting from a series of social, economic, public health, and medical victories over disease, might very well be considered the crowning achievement of the past century and a half, the increased length of life coupled with the shifts in dependency ratios present formidable long-term challenges.

The pace of the change is accelerating. In countries where the Demographic Transition began earlier, the process was slower: it took France 115 years for the proportion of the age group 65 and older to increase from 7 to 14% of the total population, and the United States will soon have completed this same increase in 69 years. But in countries that started the transition later, the process is occurring much more rapidly: Japan took 26 years to go from 7 to 14% age 65 and older, while China and Brazil are projected to require just 24 years.

Sometime around the year 2020, for the first time ever, the number of people age 65 and older in the world is expected to exceed that of children under the age of 5. Around the middle of the twentieth century, the under-5 age group constituted almost 15% of the total population and the over-65 age group 5%. It took about 70 years for these two to reach equal proportions. But demographers predict it will take only another 25–30 years for the 65 and older age group to equal about 15% and be about double the number of children under age 5. By the middle of their careers, medical students in most countries should expect to be practicing in far older populations. Preparations for these changes need to begin decades in advance, and the costs and penalties for delay can be very high. Although some governments have started planning for the long term, many, if not most, have yet to begin.

HISTORICAL

Population aging around the world in recent decades has followed a broadly similar pattern, starting with a decline in infant and childhood mortality that precedes a decline in fertility; at later stages, mortality at older ages declines as well. Declining fertility began as early as the beginning of the nineteenth century in the United States and France and extended to the rest of Europe and North America and parts of East Asia by the middle of the twentieth century. Since World War II, fertility declines have started in all other world regions. In fact, more than half the world’s population now lives in countries or provinces with fertility rates below the replacement level of just over two live births per woman. Mortality rates also began to change, relatively slowly at first, in Western Europe and North America during the nineteenth century. At first, changes were most evident at the youngest ages. Improvements in water supply and sewage handling, as well as in nutrition and housing, accounted for most of the improvement before the 1940s, when antibiotics and vaccines and increasing education of mothers began to make a major impact. Since the middle of the twentieth century, the “Child Survival Revolution” has spread to all parts of the world. Children almost everywhere in the world are much more likely to reach late middle age now than in previous generations.

Especially since around 1960, mortality at older ages has improved steadily. This improvement has been primarily due to advances in care of heart disease and stroke and in control of conditions like hypertension and hypercholesterolemia that lead to circulatory diseases. In some parts of the world, smoking rates have declined, and these declines have led to lower incidence of many cancers, heart disease, and stroke.

The initial decline in fertility resulted in older age groups becoming a larger fraction of the total population. Declines in adult and old age mortality contributed to population aging in the later stages of the process. Life expectancy at birth—the average age to which someone is expected to live, under prevailing mortality conditions—has been calculated at around 28 years in ancient Greece, perhaps 30 years in medieval Britain, and less than 25 years in the colony of Virginia in North America. In the United States, life expectancy climbed slowly during the nineteenth century, reaching 49 years for white women by 1900. White men had a life expectancy 2 years lower than that for white women, and black Americans had a life expectancy 14 years lower than did white Americans in 1900. By the early twenty-first century, life expectancy in the United States had improved dramatically for all, with the sex gap wider and the racial gaps narrower than at the beginning of the century: 76 years for white men in 2006; 81 years for white women; and 70 and 76 years for black men and women, respectively. However, although the United States had a relatively high life expectancy compared to other high-income countries around 1980, almost all such countries have in the interim exceeded the United States in life expectancy. Female life expectancy, especially for whites in the United States, has done particularly poorly, and this has been attributed to relatively high rates of lifetime smoking.

At later stages of the demographic transition, mortality declines at the oldest ages, leading to increases in the 65 and older population, and the oldest old, those older than age 85 years. Migration can also affect population aging. An influx of young migrants with high birth rates can slow (though not stop) the process, as it has in the United States and Canada; or the out-migration of the young leaving older people behind can accelerate aging at the population level, as it has in many rural areas of the world.

REGIONAL AGING—NUMBERS AND PERCENTAGES OLDER THAN AGE 60 YEARS

Regions of the world are at very different stages of the demographic transition (Fig. 93e-1). Of a world population of 6.8 billion in 2012, approximately 11% were older than age 60 years, with Japan (32%) and Europe (22%) being the oldest regions (Germany and Italy 27% each) and the United States having 19%. The percentage of the population older than age 60 years in the United States has remained lower than in Europe, due both to modestly higher fertility rates and to higher rates of immigration. Asia has about 10% older than age 60 years, with the population giants close to the average—China (12%), Indonesia (9%), and India (7%). Middle Eastern and African countries have the lowest proportions of older people (5% or lower).

FIGURE 93e-1   Percentages of national populations age 60+, in 2010. (From the U.S. Census Bureau, International Database. StatPlanet Mapping Software.)

Based on estimates from the United Nations Population Division, 809 million people were age 60 years or older in 2012, of whom 279 million lived in more developed countries and 530 million in less developed countries (as classified by the United Nations). The countries with the largest populations of those age 60 and older were China (181 million), India (100 million), and the United States (60 million).

NUMBERS—POPULATION SIZE PROJECTIONS

Population projections make use of expected fertility, mortality, and migration rates and should be regarded as uncertain when applied 40 or more years in the future. However, the population that will be age 60 and older in 2050 have all been born and survived childhood in 2014, so uncertainty about their numbers (as distinct from their proportion of the total population) is not great. Comparing the maps of the world in 2010 (Fig. 93e-1) and 2050 (Fig. 93e-2), it is apparent that the middle- and low-income countries in Latin America, Asia, and much of Africa will soon be joining the “oldest” category. In less than four decades between 2012 and 2050, the United Nations Population Division projects that the world population age 60 and older will more than double to 2.03 billion, with the least developed regions more than quadrupling. China’s 60+ population is projected to reach 439 million, India’s 323 million, and the United States’s 107 million. Over the same period, the median age of the world’s population is expected to increase by 10 years.

FIGURE 93e-2   Percentages of national populations age 60 +, in 2050 (projections). (From the U.S. Census Bureau, International Database. StatPlanet Mapping Software.)

Current global life expectancy at birth is estimated to be 65.4 for men and 69.8 for women, with the comparable figures for the more developed region being 73.6 and 80.5 years. Life expectancy in the least developed countries averaged only 57.2 for women and 54.7 for men. Life expectancy at birth is heavily influenced by infant and child mortality, which is considerably higher in poor countries. At older ages, the gap between rich and poor nations is narrower; so while women who have reached age 60 in wealthy countries can expect 23.7 more years of life on average, women at age 60 in poor countries live 16.8 years on average—a significant difference but not so stark as the difference in life expectancy at birth. At the lowest levels of per capita gross national product (GNP), life expectancy shows a powerful positive association with this measure of economic development, but then the slope of the relationship flattens out; for countries with average incomes above about $20,000 per year, life expectancy is not closely related to income. At each level of economic development, there is significant variation in life expectancy, indicating that many other factors influence life expectancy.

Japan, France, Italy, and Australia currently have some of the highest life expectancies in the world, while the United States has lagged behind other high-income countries since about 1980, especially in the case of white women. The causes of this lag are being explored, but the cumulative number of years that people have smoked tobacco by the time they reach older ages and the prevalence of obesity appear to play important roles.

GROWTH OF THE OLDEST OLD POPULATION—THOSE OVER AGE 85

A modern feature of population aging has been the almost explosive growth of the age group known as the oldest old, variously defined as those over age 80 or age 85. This is the age group with the highest burden of noncommunicable degenerative disease and related disability. Thirty years ago, this group attracted little attention because they were hidden within the overall older population in most statistical reports; for example, the U.S. Census Bureau merged them into a 65+ category. The reduction of mortality at older ages coupled with larger birth cohorts surviving into old age led to the rapid growth of the oldest old. This age group is predicted to grow at a significantly higher rate than the 60+ population, and one estimate has the current 102 million age 80+ increasing to almost 400 million by 2050 (Table 93e-2). Projected increases are astounding: China’s 80+ population might increase from 20 to 96 million, India from 8 to 43 million, the United States from 12 to 32 million, and Japan from 9 to 16 million. The numbers of centenarians are increasing at an even faster rate.

THE FUTURE OF LIFE EXPECTANCY

The members of the population who could potentially become age 80 and older in 2050 are already alive today. The actual numbers of people who will be age 80 and older in 2050 will therefore depend almost solely on adult and old age mortality rates over the next 35 years. The history of the decline of mortality suggests that improvements in the standard of living, including increased and improved education and improved nutrition, coupled with improvements in public health stemming from an understanding of the germ theory of disease initially led to the decline in mortality, with medical achievements such as antibiotics and improved understanding of risk factors for cardiovascular and circulatory diseases becoming factors only in the post–World War II period; the largest strides in cardiovascular disease came only in more recent decades. The improvements in educational attainment of succeeding generations have been credited in large part for improvements in child mortality during the past century, because educated mothers are especially likely to understand and take advantage of measures to reduce infection. The effects of continuing progress will likely be seen in coming decades as well, because educational attainment is associated with improved health and survival at older ages. Countries vary in the extent to which the “future elderly” cohorts will be more educated. China in particular will have a much more educated elderly population in 2050 (with more than two-thirds of the 65+ population having completed secondary school) than it did in 2000 (when only 10% of older people had a secondary education). In the United States and other rich nations, this change has largely taken place already; future changes in educational attainment of the elderly population will be less dramatic.

Holding aside the possibility of new infectious diseases ravaging populations as AIDS did in some African countries, debates about future life expectancy revolve around the balance and influence of risk factors such as obesity; the possibility of reducing the deaths from current killers such as cancer, heart disease, and diabetes; whether there is some natural limit to life expectancy; and the distant though nonzero possibility that science will find a way to slow the basic processes of aging.

While some have posited natural limits to human life expectancy, the limits have been surpassed with some regularity, and at the very oldest ages in the leading countries with the highest life expectancy, there appears to be little evidence of any approaching asymptote. Indeed a surprising discovery was that life expectancy in the leading country over the last century and a half, with different countries taking the lead in different epochs, could be represented almost perfectly by a straight line, with the increase for females showing a steady and astonishing increase of three months per year or 2.5 years per decade (Fig. 93e-3). No single country kept that pace of improvement the entire time, but this trend calls into question the notion that improvement must slow down, at least in the near future.

FIGURE 93e-3   Life expectancy in most advanced nations, 1800–2000, females. (From J Oeppen, JW Vaupel: Science 296:1029, 2002.)

There remains a great deal of diversity in health conditions both among and within national populations. There is nothing inevitable about the mortality transition—in several African countries, the prevalence of AIDS has been high enough to cause life expectancy to fall below the levels of 1980. Though none has so far reached a scale to rival the AIDS epidemic, periodic outbreaks of new influenza viruses or “emerging infectious” agents remind us that infectious diseases could again come to the fore. Progress against chronic disease is also reversible: In Russia and some other countries that formed part of the Soviet Union before 1992, life expectancy for men has been declining, now reaching levels below those of men in South Asia. Much of the gap between Russian and Western European men is explainable by much greater heart disease and injuries among the former.

DEPENDENCY AND CAREGIVING RATIOS

Ratios of different age groups provide useful though crude indicators of potential demands on resources and resource availability. One set of ratios, known variously as dependency or support ratios, compare the age groups who are most likely to be in the labor force with the age groups typically dependent on the productive capacity of those working—the young and the old, or just the old. A commonly used ratio is the number of persons age 15–64 per persons age 65 and older. Even though many in some countries do not enter the labor force until significantly older than age 15, retire before age 65, or work past age 65, the ratios do summarize important facts, especially in countries where financial support for the retired comes partially or mainly from those currently in the labor force through either a formal pension system or through informal support from the family. While many countries still have very basic pension systems with incomplete coverage, in Europe public pensions are quite generous, and these countries face dramatic changes in their ratios of working age to older populations. Over the next 40 years, Western Europe faces a drop in the ratio from 4 to 2. In other words, while in crude terms there are today 4 workers supporting the pensions and other costs of each older person, by 2050 there will only be 2. China faces an even steeper drop from 9 persons of working age to only 3, while Japan declines from 3 to just 1. Even in India, projected to become the most populous country, the decline is quite steep from 13 to 5.

The dramatically declining number of workers per older person (however determined) is at the crux of the economic challenge of population aging. The extra years of life that can be considered the crowning achievement in medicine and public health of the last 150 years have to be financed. The economic model of the life cycle assumes that people are economically productive for a limited number of years and that the proceeds of their work during those years have to be smoothed over to finance consumption during less economically productive ages, either within families or by institutions such as the state in order to provide for the young, the old, and the infirm. There are only so many ways to meet the challenge of an extended period of dependency, including increasing the productivity of those in the labor force, saving more, reducing consumption, increasing the number of years worked by increasing the age of retirement, increasing the voluntary nonmonetary productive contributions of the retired, and immigration of very large numbers of young workers into the “old” countries. Pressures to increase retirement ages in industrialized countries and to reduce benefits are increasing. But no single one of these measures can bear the full load of adaptation to population aging, since the changes would have to be so severe and disruptive as to be politically impossible. More likely, there will be some combination of these measures.

Population health and the ability to function at work and in everyday life interact with these population ratios in significant ways. The physical and cognitive capacity to continue to work at older ages is crucial if the age of retirement is raised. Similarly, caregiving often requires significant physical and emotional stamina. Further, healthier older populations require less caregiving and medical services. Just two decades ago, the prevalent view of aging was highly pessimistic. Epidemiologists held that while modern medicine could keep older people alive, nothing much could be done to prevent, delay, or significantly treat the degenerative chronic diseases of aging. The result would be that more and more older people with chronic diseases would be kept from dying, with the consequent piling up of the older people disabled by chronic disease. Surprisingly, between 1984 and about 2000, the prevalence of disability in the 65+ population in the United States declined by about 25%, suggesting that in this respect, aging was more plastic than had been previously believed (Fig. 93e-4). All the causes of this significant shift in disability are not yet understood, but rising levels of education, improved treatment of cardiovascular diseases and cataracts, greater availability of assistive devices, and less physically demanding occupations have been found to contribute. One calculation showed that if the rate of improvement could be maintained until 2050, that the numbers of disabled in the older population could be kept constant in the United States despite the aging of the baby boomers and the older population itself growing older. Unfortunately, the rapid increase in obesity rates could slow and perhaps even reverse this most positive trend. Because of the absence of comparable data in other countries, it is less certain whether the same pattern of improvement in disability rates (with recent deceleration) is occurring outside of the United States. Using estimates and projections of disease prevalence from the Global Burden of Disease Study, the global population of those “dependent and in need of care” is projected to rise from about 350 million in 2010 to over 600 million in 2050. Worldwide, about half of the older persons in need of care (two-thirds of the dependent population age 90 and above) suffer from dementia or cognitive impairment. A global network of longitudinal studies on aging, health, and retirement is now providing comparable data that may allow more definitive projections on disease and disability trends in the future. One estimate (World Alzheimer’s Report 2010) projected that the 36 million people with dementia worldwide in 2010 would increase to 115 million by 2050. The largest increases would occur in low- and middle-income countries where about two-thirds already live. The estimated costs were $604 billion in 2010 with 70% occurring in North America and Western Europe. A 2013 study using a nationally representative U.S. sample found that annual dementia costs could be as high as $215 billion. Direct costs of dementia care exceeded the direct costs for either heart disease or cancer. Given the age-associated prevalence of dementia and the expected increase in the older population, coupled with the associated decline in family members able to provide care, countries need to plan for a pandemic of individuals requiring long-term care.

FIGURE 93e-4   Disability prevalence, various years 1982–2005, by age group over 65, United States. (Adapted from KG Manton et al: Proc Natl Acad Sci U S A. 103:18374, 2006.)

Population aging, and related demographic changes including changes in family structure, could affect the “supply side” of long-term care as well as the demand for care and health care. In every country, long-term care of the disabled and the chronically ill relies heavily on informal, typically unpaid caregivers—usually spouses or children; and increasingly in more developed countries, caregivers for the oldest old are in their 60s and early 70s. Although there are many men who provide care, on the population level, informal caregiving is still mainly done by women. Because women live longer than men, lack of a spousal caregiver is especially likely to be a problem for older women. Both men and women have fewer children on whom they can call for informal caregiving, because of the worldwide decline in fertility rates. An increasing proportion of older men in Europe and North America have spent much or all of their adult lives apart from their biological children. Lower fertility rates, delayed marriage, and increasing divorce rates mean that people approaching old age may be less likely to have close ties with daughters and daughters-in-law—the adults who have in the past been the most common caregivers apart from spouses. Adult women who in the past have provided uncompensated care (and much other essential volunteer work) are now more likely than in the past to be working for pay and thus have fewer hours to devote to the unpaid roles.

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