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Demographic ageing designates an increase in the proportion of old people in a population. Hitherto, it has essentially affected the countries of the North, where fertility and mortality have decreased considerably, but it is beginning to affect the countries of the South, and is set to be one of the major social changes for humankind in the XXIst Century.
Demographic ageing is a collective phenomenon, unlike biological ageing which is specific to each individual as they grow older. It results from a decrease in fertility and from a lengthening of the average length of life. The former causes the base of the age pyramid to grow narrower this is called ' bottom-up ageing' and the latter causes the pyramid to grow longer and wider at old ages this is called 'top-down' ageing. Demographic ageing is a consequence of demographic transition. It is irreversible unless we return to the old demographic situation of high fertility and high mortality. It affects all of the world's countries, and it will be faster in the countries of the South than in the countries of the North, since the transition is taking place faster there.