Aging of Population

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The aging of population (also known as demographic aging and population aging) is a term that is used to describe shifts in the age distribution (age structure) of a population toward people of older ages. A direct consequence of the ongoing global fertility transition (decline) and of mortality decline among people of older ages, population aging is expected to be among the most prominent global demographic trends of the twenty-first century. Population aging is progressing rapidly in many industrialized countries, but developing countries whose fertility declines began relatively early also are experiencing rapid increases in the proportion of elderly people. This pattern is expected to continue, eventually affecting the entire world.

Population aging has many important socioeconomic and health consequences, including an increase in the old-age dependency ratio. It presents challenges for public health (in particular, the increasing burden of health care costs on national budgets) as well as for economic development (such as the shrinking and aging of the labor force and the nonviability of pay-as-you-go social security systems).

Defining and Measuring

Because the study of population aging often is driven by concern about the burden it imposes on retirement systems, the aging of population often is measured by increases in the percentage of people in the retirement ages. The definition of retirement ages varies, but a typical lower cutoff number is 65 years. A society is considered relatively old when the proportion of the population age 65 and over exceeds 8 to 10 percent. By this standard the proportion of elderly people in the United States was 12.6 percent in 2000, compared with only 4.1 percent in 1900; it is projected to increase to 20 percent by the year 2030.

A related measure of population aging is the elderly dependency ratio (EDR): the number of individuals of retirement age divided by the number of those of working-age. For convenience working age may be assumed to start at 15 years, although increasing proportions of individuals pursue their education beyond that age, remaining financially dependent (on the state or, increasingly, their parents) or borrowing against their own future incomes. The ratio of the elderly dependent population to the economically active (working) population also is known as old-age dependency ratio, age-dependency ratio, or elderly dependency burden and is used to assess intergenerational transfers, taxation policies, and saving behavior.

Another indicator of the age structure is the aging index (sometimes referred to as the elder–child ratio), which is defined as the number of people age 65 and over per 100 youths under age 15. In 2000 only a few countries (Germany, Greece, Italy, Bulgaria, and Japan) had more elderly persons than youths (that is, an aging index above 100). By 2030, however, the aging index is projected to exceed 100 in all developed countries, and the indexes for several European countries and Japan are expected to exceed 200. In the United States the index was 57 in 2000; by 2030 it is projected to rise to 109. Aging indexes are much lower in developing countries than in the developed world, but the proportional rise in the aging index in developing countries is expected to be greater than that in developed countries.

These indicators of population aging are head-count ratios (HCRs); that is, they simply indicate the number of individuals in large age categories. These indicators fail to take into account the age distribution within the large categories, in particular among the elderly. When the fertility and mortality trends responsible for population aging have been fairly regular over time, population growth is positively correlated with age: The oldest age groups are growing fastest. This implies that if the proportion of the population over age 65 is increasing, within that 65-and-over population the proportion over age 80, for example, is also increasing. Health, financial situation, and consumption patterns may vary greatly between 65-year-olds and 80-year-olds, but this heterogeneity in the elderly population is concealed in simple ratios.

Increasing attention is paid to the "oldest olds" (typically defined as persons age 80 and over), a category that is expanding rapidly. The number of centenarians, a longtime subject of curiosity, is growing even faster: Estimated at 180,000 worldwide in 2000, it could reach 1 million by 2030.

The second class of indicators of population aging include the standard statistical measures of location: the median, mean, and modal ages of the population. The median age–the age at which exactly half the population is older and half younger–is the most widely used indicator. For the year 2000 the median age in the United States was 36 years, a typical value for most developed countries. The median age for Africa was 18 years. Because it is more sensitive to changes at the right-hand tail of the age distribution (the oldest old ages), the mean age of population may be preferred to the median age in studying the dynamics of population aging.

Because population aging refers to changes in the entire age distribution, any single indicator may be insufficient to measure it. The age distribution of a population is often very irregular, showing scars of past events (wars, depressions, etc.), and cannot be described by only one number without a significant loss of information. Changes in the age distribution also may occur in a very irregular fashion over the age range, hence much information would be lost in a single-index summary.

A more adequate approach to describing population aging is through a set of percentiles. Alternatively, a graphical approach may be used that involves analyzing population pyramids. Demographers commonly use population pyramids to describe both age and sex distributions of populations. Youthful populations are represented by pyramids with a broad base of young children and a narrow apex of older people, and older populations are characterized by more uniform numbers of people in the age categories.

Demographic Determinants

To understand the demographic factors that cause population aging, demographers often refer to the stable population model. This model assumes that age-specific fertility and mortality rates remain constant over time, resulting in a population with an age distribution that eventually ceases to change: It becomes "stable." Conversely, the model suggests that in a population closed to migration any change in age structure, population aging in particular, can be caused only by changes in fertility and mortality rates.

The influence of changes in fertility rates on population aging may not be obvious at first sight. With everything else held constant, however, a fertility decline reduces the size of the most recent birth cohorts relative to the previous birth cohorts, reducing the size of the youngest age groups relative to that of the older ones.

The effects of changes in mortality rates on population aging appear more intuitive but are in fact ambiguous. Reductions in mortality rates do not necessarily contribute to population aging. More specifically, declines in the mortality rates of infants, children, and persons younger than the population's mean age tend to lower that mean age. Clearly, a reduction of neonatal mortality–death in the first month of life–adds individuals at age zero and thus should lead to the same alleviation of population aging that an increase in childbearing does.

Population aging thus is related to the demo-graphic transition, the processes that lead a society from a demographic regime characterized by high rates of fertility and mortality to one characterized by lower fertility and mortality rates. In the course of this transition, the age structure is subjected to different influences. In the typical sequence the transition begins with successes in preventing infectious and parasitic diseases that most benefit infants and young children. The result is an improvement in life expectancy at birth. Fertility, however, tends to remain unchanged, thus producing large birth cohorts and an expanding proportion of children relative to adults. Other things being equal, this initial decline in mortality generates a younger population age structure.

After initial and sometimes very rapid gains in infant and child mortality have been achieved, further mortality declines increasingly benefit people of older ages and eventually are accompanied by fertility declines. Both changes contribute to a reversal of the early effect of mortality decline on the age structure, and this synergy is known as the double aging process. Most developed countries today are experiencing such a process, but further analysis suggests that their history of declining mortality is the dominant factor in current aging.

Mortality declines continue in these countries: Indeed, the decrease in mortality rates among those age 85 years and over has accelerated since the 1950s. This latest phase of mortality decline, which is concentrated in the older age groups, is becoming an important determinant of population aging, particularly among women.

The rate of population aging may be modulated by migration. Immigration marginally slows population aging (in Canada and Europe, for example) to the extent that immigrants are younger than the population average and have higher fertility than do the native-born. However, emigration of workingage adults accelerates population aging, a phenomenon that can be observed in some Caribbean countries. Population aging in those countries also is accelerated by the immigration of elderly retirees from other countries and the return migration of former emigrants who are above the average population age.


The current level and pace of population aging vary widely by geographic region and usually within regions as well, but virtually all nations were experiencing growth in the number of elderly residents at the beginning of the twenty-first century (for selected regions and countries, see Table 1). The proportion of the world population age 65 and over increased from 5.2 percent in 1950 to 6.9 percent in 2000. In Europe, however, the proportion was 14.7 percent in 2000. The highest proportions used to be found in Northern Europe (e.g., 10.3 percent in Sweden in 1950), but by 2000 they were in Southern Europe (e.g., 18.1 percent in Italy).

The proportions of elderly people are lower outside of Europe, with the notable exception of Japan, where this figure increased from 4.9 percent in 1950 to 17.2 percent in 2000. The age structure of the United States continues to be influenced by the large birth cohorts of the baby boom (people born from 1946 through 1964) who are not yet age 65. The proportion of the elderly population in the United States, which was 12.3 percent in 2000, remains low compared to the average in other developed countries.

Population aging has the following notable features:

  1. The most rapid growth occurs in the oldest age groups: the oldest old (80-plus or 85-plus years) and centenarians (100-plus years) in particular. In other words, population aging is becoming "deeper," with a disproportionately rapid accumulation of particularly old and frail people.
  2. Population aging is particularly rapid among women, resulting in "feminization" of population aging (because of lower mortality rates among women). For example, in the United States in the population 65 years and older in 2000 there were 20.6 million women and 14.4 million men, or a ratio of 143 women for every 100 men. The female-to-male ratio increases with age, reaching 245 for persons 85 and over.
  3. Another consequence of lower female mortality is the fact that almost half of older (65-plus years) women (45%) in 2000 were widows, living without spousal support.
  4. Population aging also causes changes in living arrangements, resulting in increasing numbers of older people living alone (about 30% of all noninstitutionalized older persons in 2000 lived alone in the United States).
  5. Because older persons usually have lower income and a higher proportion of them live below the poverty line, population aging is associated with poverty, particularly in developing countries.

Twenty-First Century Projections

Population aging in the future will depend on demo-graphic trends, but most demographers agree that the fertility and mortality changes that would be required to reverse population aging in the coming decades are very unlikely to occur. According to the 2000 United Nations medium population projections, population aging in the first half of the twenty-first century should exceed that of the second half of the twentieth century. For the world as a whole, the elderly (65-plus) will grow from 6.9 percent of the population in 2000 to a projected 19.3 percent in 2050 (Table 1). In other words, the world average would be higher in 2050 than the current world record.

All regions are expected to see an increase, although it should be milder in some regions, such as Africa, where the projected increase is from 3.3 percent in 2000 to 6.9 percent in 2050. However, in Latin America and the Caribbean the increase is projected to be from 5.4 percent in 2000 to 16.9 percent in 2050, higher than the current European average. The projected increase is even more spectacular in China: from 6.9 percent in 2000 to 22.7 percent in 2050.

Although population aging thus is becoming a worldwide phenomenon, the most developed countries


probably will continue to be the forerunners. The United Nations projections for 2050 suggest that there will be 29.2 percent of elderly persons in the European population as a whole but more than 30 percent in a number of individual European countries (such as Italy) and perhaps as much as 36.4 percent in Japan. Again, the projected increase appears less dramatic in the United States: from 12.3 percent in 2000 to 21.1 percent in 2050.

There is uncertainty in any projection, but it is important to note that previous population projections underestimated rather than overstated the current pace of population aging. Before the 1980s the process of population aging was considered to be a consequence of fertility decline alone, and it was predicted that the pace of population aging would decrease after stabilization of fertility rates at a low level. The rapid decline in old-age mortality that was observed in developed countries in the last decades of the twentieth century significantly accelerated population aging. At the beginning of the twenty-first century, old-age mortality trends are becoming the key demographic component in projecting the size and composition of the world's future elderly population.

Current and future uncertainties about changing mortality may produce widely divergent projections of the size of tomorrow's elderly population. For example, the U.S. Census Bureau's middle-mortality series projection suggests that in the United States there will be 14.3 million persons age 85 and over in the year 2040, whereas the low-mortality (high life expectancy) series implies 16.8 million. Alternative projections, using assumptions of lower death rates and higher life expectancies, have produced estimates ranging from 23.5 million to 54 million persons age 85 and over in 2040.

Social and Economic Implications

Although population aging represents a success story for humankind (survival to old ages has become commonplace), it also poses profound challenges to public institutions that must adapt to a changing age structure.

The first challenge is associated with the marked increase in the older retired population relative to the shrinking population of working ages, which creates social and political pressures on social support systems. In most developed countries rapid population aging places strong pressure on social security programs. For example, the U.S. social security system may face a profound crisis if radical modifications are not enacted. Cuts in benefits, tax increases, massive borrowing, lower cost-of-living adjustments, later retirement ages, and combinations of these elements are being discussed as the painful policies that may become necessary to sustain pay-as-you-go public retirement programs such as Medicare and Social Security. Privatization and shift to a funded scheme in retirement programs are also considered as potential options to cope with population aging.

Population aging also presents a great challenge for health care systems. As populations age, the prevalence of disability, frailty, and chronic diseases (Alzheimer's disease, cancer, cardiovascular and cerebrovascular diseases, etc.) is expected to increase dramatically. Some experts are concerned that human society may become a "global nursing home" (Eberstadt 1997).

The aging of the population is a global phenomenon that requires international coordination of national and local actions. The United Nations and other international organizations have developed recommendations that are intended to mitigate the adverse consequences of population aging. These recommendations include reorganization of social security systems; changes in labor, immigration, and family policies; promotion of active and healthy lifestyles; and more cooperation between governments in resolving the socioeconomic and political problems posed by population aging.

On the positive side, the health status of older people within a given age group has been improving over time. More recent generations have a lower disease load. Older people can live vigorous and active lives until a much later age than they could in the past and, if encouraged to be productive, can be economic contributors to society. Moreover, current intensive biomedical anti-aging studies may further extend the healthy and productive period of human life in the future.

See also: Age Structure and Dependency; Fertility, Below-Replacement; Life Span; Mortality Decline; Oldest Old; Second Demographic Transition.


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Leonid A. Gavrilov

Patrick Heuveline