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in population studies, the proportionate numbers of persons in successive age categories in a given population. Age distributions differ among countries mainly because of differences in the levels and trends of fertility. A population with persistently high fertility, for instance, has a large proportion of children and a small proportion of aged persons. A population, such as that of France, in which fertility has been low for a long time, has a smaller proportion of children and a larger proportion of aged persons. Changes in fertility have an immediate effect on numbers of children, but many years must pass before the change affects the numbers above childhood. Thus, a population that has experienced a recent decline in fertility tends to have relatively small numbers both of children and of aged persons and a large proportion of adults in the middle ages.