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Population Aging - Age Distribution Of A Population

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As death rates drop and life expectancy increases, the older population increases partly because life expectancy goes up somewhat at the older age. However, the most important impact on aging is due to the fact that lower mortality increases the probability of surviving to old age. For example, in 1900 the life expectancy in the United States was lower than in almost every country in the world today. At birth a male could have expected to live an average of only 46.3 years, while the average was 48.3 for females. These life expectancies were about the same as England at the time, but lower than most Scandinavian nations. At this level of mortality, a male baby had only a 36 percent chance of surviving from birth to age sixty-five, and a female baby had a 41 percent chance of living that long. For those who reached 65, the pattern of mortality in 1900 produced a life expectancy at age 65 of 11.5 additional years for males, and 12.2 additional years for females. In that year only 4.1 percent of the U.S. population was age sixty-five and older, comparable to the countries of sub-Saharan Africa and the Middle East as of the year 2000.

Significant improvements in mortality occurred in the United States and other developed countries between 1900 and 1950, with female life expectancy in the United States increasing from 48.3 to 71.1 years. In concert with that, the percentage age sixty-five and older doubled to 8.2 in 1950. By the end of the twentieth century, life expectancy at birth for females in the United States had improved to 79.2 years—30.9 years more than at the turn of this century. Along with this, the percentage age sixty-five and older increased to 12.7 of the total. The century of improvement in life expectancy more than doubled the proportion of female babies surviving from birth to age sixty-five and has been a major reason for the increase in the number of older persons in the United States.

It is now commonplace for people to reach old age; so much so that long life is virtually taken for granted in richer nations. Furthermore, on reaching age sixty-five, females in the United States at the end of the twentieth century could expect to live an additional 19.2 years on average—an improvement of 7.0 years more beyond age 65 than was true a hundred years earlier, in 1900 (Anderson). This translates into more than a doubling of the proportion of women age sixty-five who will still be alive at age eighty. In 1997 in the United States, for example, more than two-thirds (68 percent) of the women alive at age sixty-five will still be alive at age eighty. The situation is not quite so favorable for males, but it has been improving for them, too.

Although declining mortality will always lead to an increase in the number of older people, the percentage age sixty-five and older will increase noticeably only if fertility also declines, as it always has historically in the context of the demographic transition (from low life expectancy and high fertility to high life expectancy and low fertility). Stable population models (Coale and Demeny) can be used to illustrate the percentage of the population that would be age sixty-five and older if a population maintained different combinations of mortality and fertility over time. For example, a country whose life expectancy was only thirty years would have 3.9 percent of the population age sixty-five and older if the total fertility rate (TFR) were five children per woman, and it would drop to 2.8 percent if the TFR went up to six. On the other hand, at a TFR of four or below at this low life expectancy, the population would be depopulating (because of fewer births than the number of deaths) so the percentage age sixty-five and older would be temporarily high, and then everybody would die off.

Stable population models show us that as life expectancy increases from thirty years to sixty years in a population where fertility remains at five children per woman, the percent age sixty-five and older goes down slightly. This actually happened in Mexico. Mortality was declining in Mexico between 1950 and 1970, but fertility had not begun to decline and so the older population was declining as a percent of the total population. However, as mortality continues to decline, eventually it gets low enough that the percentage representing the older population begins to increase even if fertility does not change. At any given level of life expectancy, the lower the level of fertility, the higher is the percentage of the population that is sixty-five or older. The typical path followed by the percentage of the population that is older as a country passes through the demographic transition is that the percentage goes from about 2–3 in premodern societies to 20 percent or even higher as life expectancy reaches eighty years and as fertility drops below replacement level.

The impact of migration on the growth of the older population is more complex than the effect of either mortality or fertility. Younger people are more likely to migrate than the elderly, but there is no biological limitation to migration and so it is that, although in general migration tends to leave the older population behind, there are other times when older people move disproportionately to specific areas (such as Florida in the United States) in search of "amenity-rich communities with sunnier, warmer and recreationally more enjoyable environments" (Rogers, 1992, p. 3). Migration affects population aging most by what older people do not do—they do not migrate very much. Throughout the world, older people tend to age in place. As a consequence, the process of urbanization, which occurs in concert with the demographic transition, has left older people abandoned in the countryside by their children who migrate to the cities in search of work. In general, outmigration tends to increase the percentage of the population that is older in a region, whereas immigration has the opposite effect.

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