The structure of an ageing population such as that in Britain has been determined largely by fluctuations in the birth rate and the downward trend in infant mortality; the impact of improvements in mortality in adult life has been small (Benjamin, this volume). It is, therefore, possible to make accurate minimal estimates of the numbers of the elderly in Britain over the next 20 years: given only moderate improvements in mortality, we can expect little or no change in the overall numbers of those aged 65 and over, but a 16% increase in those aged 75 and over and a 48% increase in those aged 85 and over (O.P.C.S., 1983). We shall use the British population for illustrative purposes, but human ageing is a universal process and population ageing is a phenomenon about which nations can learn through the experience of others. Perhaps nowhere is the issue more vivid than in the dilemmas facing the policy-makers of China who seem to have achieved the impossible in rigorous control of the birth rate (Keyfitz, 1984). The future well-being of the Chinese nation depends on the skill with which its population structure can be titrated against its resource production and consumption. If successful, this endeavour will truly represent a giant step for mankind in rational environmental control.

Age-specific incidence rates for some important age-associated diseases follow a power-law relationship to age. This is so for most adult cancers, although the curves for cervix uteri and breast cancers in the female show a point of inflection towards a less steep increase with age in later life (Doll, 1970).