The aging of a country’s population simply means that an increasing proportion of the total population is elderly, sometimes defined as age 60 and above and other times as age 65 and above. Population aging is caused primarily by fertility decline. As smaller birth cohorts grow up to be children and then adults, they constitute much smaller proportions of the total population than did the children of earlier high fertility periods. The elderly then constitute a correspondingly higher proportion of the population. In addition, when a population has already achieved low mortality, further mortality improvements primarily extend the survival of older age groups, which tends to increase the aging of the population