The ageing of population is a twentieth century phenomenon, but it is not one that is likely to be reversed. Indeed a historical perspective on world population trends will convince us that the ageing of populations throughout the world is inevitable. It is therefore something to be accommodated and planned for, not to be fought against. Ageing is inevitable simply because of the finite nature of the earth's resources, which implies that continued population growth is not possible. The only civilized way to bring population growth to an end is to balance low mortality rates with low fertility rates: the alternative is high mortality and fertility, the condition prevailing through most of mankind's history. It is a demographic fact of life that populations with low mortality and fertility will, after these trends have been established for some time, have a high proportion of elderly people.

Thus it can be said that the medical and public health advances of the past two centuries have guaranteed that populations will age. This is not so much because people are living longer (though in popular understanding this is often believed to be the main cause of ageing) as because lowered mortality means accelerated rates of population growth (which cannot long be sustained) unless birth rates fall. And it is the fall in birth rates which is the crucial determinant of ageing, through its effect in undercutting the base of the age pyramid (U.N. 1956; Coale 1956).

A few moments' thought should convince the reader that mortality declines in ASEAN countries do not directly serve as a major cause of ageing. Such declines are often heavily concentrated at the infant and childhood ages. As such, their effects are similar to those of a rise in the birth rate, resulting in more people at the base of the age pyramid. It is only after mortality declines have been in progress for a long time that they tend to be relatively greatest at the older ages, thus reinforcing the tendency for the population to age.