A by-product of this new longevity is a remarkable increase in the number of individuals living to be 100. While centenarians were rare in 1900, their numbers swelled to 32,000 by 1982, 61,000 today, and it is projected that by the middle of the next century; there will be over 600,000 individuals in the United States over the age of 100! Four out of five centenarians are women.
Success in extending life has not reduced the appetite for further gains. It has, however, brought into prominence the question of ultimate limits: Is there some species-determined limit to the number of years human beings can live?
We have only tentative-and controversial-answers to that question. The biblical reference to "three-score and ten" as the appropriate span of human life is often quoted, but the average modern American has already exceeded that limit, and there are biblical examples, from Abraham to Methuselah, of people who attained far greater ages. Some optimistic geriatricians have proposed 120 years as the potential human life span, in the absence of accident or disease. Interestingly, this age is consistent with the death, in August 1997, of Jeanne Calment in France-at 122, the longest well-documented life on record. The number 120, or something close to it, comes from comparisons across species from fruit flies to Galapagos turtles. In almost every species, the oldest age observed is approximately six times the length of time from birth to maturity. In the human case, this argues for a span of 108 or 120 years, assuming that the age of complete biological maturity is eighteen to twenty years. On the other hand, many scientists now feel that it is unlikely that there is any fixed life span limit.
More conservative predictions come from statisticians and mathematical demographers who focus not on the maximum age to which a person can live (which may or may not be fixed) but on the average age at death of the entire population. They estimate that average life expectancy at birth in the United States (now about seventy-six years) will reach eighty-three years by the year 2050 and that the practical upper limit of average human life expectancy may not be much more than that. Several lines of research lead them to this conclusion. First, as actuaries from the Social Security Administration point out, we have already experienced the great gains conferred by the widespread use of antibiotic drugs against infectious diseases; little more can be expected from this source. Second, further progress in preventing or deferring the onset of heart attacks, strokes, and cancer will be slower-and these are the major causes of death among older men and women.
Furthermore, some experts believe that even prevention or cure of cancer and heart disease would fail to take us beyond the eighty-five-year limit. Why? Because it has been observed that beginning at age twenty, human death rates for each decade are twice that of the preceding ten-year period. The same applies to regular increases in death rates among zoo animals and pets that are protected from the hazards of life in the wild. If this regularity reflects some unchanging law of nature, cutting out later life illness may not be a meaningful factor in lengthening life.
To understand the combined magnitude of these changes in life expectancy, let us think of the total population of our country as divided into five-year age groups. We begin with males and females just born, zero to four years of age, then those five to nine, ten to fourteen, and so on to the oldest old. The number of people, male and female, in each of these age groups is known. Those numbers can be represented visually by means of a bar chart, in which the length of each horizontal bar shows the millions of people in each age group, males on the left and females on the right, in each age group.
When such a chart is drawn for the years before 1900, it is shaped like a triangle or a pyramid, with the greatest numbers at the youngest ages and fewer people in each older age group. But the present and the future pattern, at least for the United States and other technologically advanced countries, is very different-more like a slightly lopsided rectangle or squat pillar than a pyramid. The rectangular shape means that infant mortality is very low, that the infectious diseases of childhood are greatly reduced, and that the impact of chronic diseases is postponed to old age. Substantial reductions in the length of the bars occur only after age sixty-five. The lopsided shape of the bars, especially after age sixty-five, shows the longer survival of women than men. Infants born in 1992 will live, on the average, to age seventy-six, but the women will live seven years longer than the men.
Not only are there more older people in our society than ever before, but those seniors are different than they used to be. The eminent gerontologist Bernice Neugarten first suggested that older people be divided into two groups, the "young-old," who are about age sixty-five to seventy-four, generally well and highly functional; and the "old-old," those over age seventy-five, who are much more likely to be frail. This latter group has also been referred to as in the "last quarter" of life. The old-old are by far the fastest-growing subset of the aging population. Of all individuals over age sixty-five in the United States in 1900, only 4 percent were over age eighty-five. Today, that proportion has reached over 10 percent, and continues to grow.