Population Aging Population aging refers to changes in the age composition of a population such that there is an increase in the proportion of older persons. Demographers use age/sex pyramids to illustrate the distribution of populations across all age groups. In 2000, Brazil has a large portion of the population in the younger ages. By 2025 and 2050, the proportion of the younger population has decreased dramatically, and the proportion in the oldest group, 85 years and older, has increased dramatically. In contrast, in 2000 Italy’s young population already constitutes a smaller portion of the total population. By 2050, persons under the age of 20 will constitute only 16.5% of the total Italian population, whereas those 60 and older will make up almost 40%. Epidemiologic transition theory highlights how changes in the causes of mortality, from infectious and parasitic diseases to chronic and degenerative diseases, result in longer life expectation for populations. Thus, the changing process of mortality is one factor affecting population aging. A second important factor is changes in the rates of fertility or births. The changing age/sex structure of Brazil is due, in part, to reductions in the average number of children born per female. Italy is among the more developed countries that already have low birth rates in 2000, as represented by the small proportion of the population in the younger age groups. A third factor affecting population aging is migration; however, the effect of migration on population aging is not as influential as that of changes in trends in population mortality and fertility. Patterns of migration into and out of nations can affect the age structure of populations. Immigrants tend to be younger, and first-generation immigrants tend to follow marriage and fertility patterns typical of their original country. Thus, a large flow of immigrants into a country can potentially shift the age structure to younger ages. A country that experiences significant out-migration may also shift to an older population structure if those migrating out are young adults and their children. All regions of the world are experiencing population aging, although the rates of increase differ across regions. Europe is the oldest region, with 21% of its population aged 60 years and over in 2006, and the rate is expected to rise to 34% in 2050. The youngest region is Africa, with only 5% of the population over the age of 60 in 2006, which will double to 10%in 2050. The proportion of older persons in both Asia and Latin America and the Caribbean regions are expected to increase from 9% to 24% between 2006 and 2050, due to both reductions in rates of fertility and increases in life expectancy. A United Nations report, World Population Aging: 1950–2050 (2002), set forth four important conclusions regarding the trend of population aging. First, population aging is unprecedented in the history of humanity. By 2050, it is predicted the worldwide number of persons aged 60 years and older will be larger than the number of persons aged 15 years and under. Second, population aging is a global phenomenon that affects persons of all ages in all nations. Third, population aging has serious implications for economic, political, social, and other areas of human life. Fourth, population aging is enduring. The trend will continue through the twenty-first century, and it is unlikely any nation will return to younger populations of the past.