Population ageing is represented by an increase in the relative number of older persons in a population and is associated with an increase in the median age of the population. The age structure of a population is determined by its mortality, fertility, and net migration experience. Although life tables and survivorship rates date from the 17th century, the development of mathematical demography is essentially a 20th-century innovation. The techniques of mathematical demography can be used to show how the age structure of a population changes with alternative transition rates.
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