13. Stationary population
The stationary oldest-old population is shaped by a constant number of persons annually reaching age 80 and then being subject to constant mortality. The following calculations are based on an annual contingent of 10,000 persons of whom 3500 are male and 6500 female - the average proportions observed in the group of thirteen countries in 1950-1990. Two mortality regimes are applied, those of 1960-70 and 1980-90 in the group of thirteen.
The stationary populations resulting from these premises are given in Table 27. The lower mortality schedule of 1980-90 adds of course very little at ages immediately after 80 but swells the ranks from 85 to 95 or so and allows a vanguard to advance in some strength past age 100. In the proportional distribution, which is illustrated by the age pyramid in Figure 26, a significant transformation is noticeable. The pyramid has lost some of its very broad base, compensated by incipient filling of the middle steps in the nineties. Women are now relatively more numerous than before beginning with age 85, the men beginning with 86. Built on an initial sex ratio 35:65, the two sides of the pyramid are not independent and a small relative loss of the male side has occurred in the 20-year period.
The centenarian situation is in these presentations overshadowed by the mass of the younger groups and requires separate accounting which is done in Table 28 and Figure 27.
The stationary centenarian population, based on the observed sex ratio 20:80, has undergone a similar development but, when the number reaching age 100 is held constant, the effect is muted. We note an overall increase in size and a penetration into the supercentenarian age.
In the 1960s, as many as 44 percent of all centenarians were still in their first year in this category. Twenty years later, the proportion has fallen to 40 and corresponding increases have accrued to ages 102 and above. Also this pyramid has lost some of its extremely broad base and begun to fill up at higher ages.
It follows from declining centenarian mortality and thus comes as no surprise that even the centenarian population has a tendency to age and that, barring a mortality increase, only an ever-increasing influx of new centenarians can "rejuvenate" it.
The stationary oldest-old population of individual countries has been calculated on the sex ratio 35:65 and the mortality regime of 1980-90 in the country concerned. They are given in Annex Table 10. The total size is of course a direct result of life expectancy at 80 but what is of interest here is the age distribution which is obtained, not in its historical context but as function of the mortality of the period. The results are summarized in Table 29 and given in Figure 28.
In all populations of the database, the majority of the people aged 80 and over are less than 85 years of age. Large differences, however, exist as can easily be noted. The percentage below 85 among all those above 80 varies from 51 in Iceland to 64 in Eastern Europe, and there is a clear tendency for decline. In the group of thirteen countries, the percentage dropped in 20 years from 62 to 55.
Updated by V. Castanova, 1 November 1999