Since it is possible to identify the age groups which are affected by changes in fertility and mortality, it is not difficult to make firm generalisations about the effects of these factors on the age structure of a population.
It has been explained earlier that changes in fertility affect the base of the age pyramid, while changes in mortality affect infants and young children in the developing countries and the older age groups in the developed countries.
No such general rules can be laid down with respect to the effects of net migration on age structure. Two important factors have to be taken into consideration for determining this effect: the age distribution of the net migrants and the volume of net migration.
If the proportion and number of young adults among the net migrants is large, the effects of aging of the population tend to get retarded. Since these young adults can participate in reproduction, they also add proportionately to the flow of births.
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If, on the other hand, the net migrants contribute mainly to the increase of those who are above the age of 30, the aging process tends to get accelerated. Thus, whether age distribution would be affected and, if so, in what direction, depends on the volume and age distribution of the net migrants.
Both international as well as internal migration can affect the age-sex structure of a population. The union territory of the Andaman and Nicobar Islands provides an apt illustration of the effect of internal net migration on age-sex distribution, in which the age-sex pyramid of this territory in 1961 has been presented.
The striking feature of this pyramid is that it is irregularly shaped and has an unusually large bulge on the male side, especially in the age group 20-34.