Increase in the proportion of adults and elderly people in a population, due to a decrease in fertility and in mortality.
Population ageing may be due to an increase in the number of old people (top-down ageing) thanks to lower mortality and longer life expectancy, but may also be caused by a deficit of young people (bottom-up ageing), due to declining fertility. In this case, ageing may occur even if the number of old people does not increase.
Population ageing is the consequence of the demographic transition. Until now, it has mainly concerned the Northern countries, where fertility and mortality have fallen sharply, though it is now starting to have an impact in the South. It is likely to be one of the major social changes affecting humanity in the 21st century.