Population aging around the world in recent decades has followed a broadly similar pattern, starting
with a decline in infant and childhood mortality that precedes a decline in fertility; at later stages,
mortality at older ages declines as well. Declining fertility began as early as the beginning of the
nineteenth century in the United States and France and extended to the rest of Europe and North
America and parts of East Asia by the middle of the twentieth century. Since World War II, fertility
declines have started in all other world regions. In fact, more than half the world's population now lives
in countries or provinces with fertility rates below the replacement level of just over two live births per
woman. Mortality rates also began to change, relatively slowly at first, in Western Europe and North
America during the nineteenth century. At first, changes were most evident at the youngest ages.
Improvements in water supply and sewage handling, as well as in nutrition and housing, accounted for
most of the improvement before the 1940s, when antibiotics and vaccines, and increasing education of
mothers began to make a major impact. Since the middle of the twentieth century, the "Child Survival
Revolution" has spread to all parts of the world. Children almost everywhere in the world are much more
likely to reach late middle age now than in previous generations.
Especially since around 1960, mortality at older ages has improved rapidly in most of the developed
countries. This improvement has been primarily due to advances in care of heart disease and stroke
and in control of conditions like hypertension and hypercholesterolemia that lead to circulatory
diseases. In some parts of the world, smoking rates have declined, and these declines have led to lower
incidence of many cancers, heart disease, and stroke.
The initial decline in fertility resulted in older age groups becoming a larger fraction of the total
population. Declines in adult and old age mortality contributed to the later stages of the process. Life
expectancy at birth—the average age to which someone is expected to live, under prevailing mortality
conditions—has been calculated at around 28 years in ancient Greece, perhaps 30 years in medieval
Britain, and less than 25 years in the colony of Virginia in North America. In the United States, life
expectancy climbed slowly during the nineteenth century, reaching 49 years for white women by 1900.
White men had a life expectancy 2 years lower than that for white women, and black Americans had life
expectancy 14 years lower than did white Americans in 1900. By the early twenty-first century, life
expectancy in the United States had improved dramatically for all, with the sex gap wider and the racial
gaps narrower than at the beginning of the century: 76 years for white men in 2006; 81 years for white
women; and 70 and 76 years for black men and women, respectively.
At later stages of the demographic transition, mortality declines at the oldest ages, leading to increases
in the 65 and older population, and the oldest old, those older than age 85 years. Migration can also
affect population aging. An influx of young migrants with high birth rates can slow (though not stop) the
process, as it has in the United States and Canada; or the out-migration of the young leaving older
people behind can accelerate aging at the population level, as it has in many rural areas of the world.
Wars and pandemic diseases such as AIDS can also change age composition by decimating particular