Most demographers are aware that the age structure of a population is produced by processes of fertility, mortality, and migration. However, the precise manner in which these processes combine to produce aging is not widely understood. Since aging is one of the major demographic phenomena of the twenty-first century in most countries of the world, a better understanding of the factors responsible for it is desirable.
Equilibrium models have generated important insights into processes of population aging. The stable population model formalized by Lotka (1939) and elaborated by Coale (1976) demonstrated that, under very non-restrictive conditions, populations that experience constant age-specific fertility rates, constant age-specific mortality rates, and zero migration will eventually achieve a constant age structure. Using this model in a comparative statics framework, one can compare the structures of populations with two different sets of demographic parameters in order to identify the eventual impact of variations in fertility and/or mortality. Typically, populations require several generations, or 60-70 years, before one stable equilibrium can be replaced by another.