The dramatic increase in average life expectancy during the 20th century ranks as one of society’s greatest achievements. Although most babies born in 1900 did not live past age 50, life expectancy at birth now exceeds 83 years in Japan—the current leader—and is at least 81 years in several other countries. Less developed regions of the world have experienced a steady increase in life expectancy since World War II, although not all regions have shared in these improvements. (One notable exception is the fall in life expectancy in many parts of Africa because of deaths caused by the HIV/AIDS epidemic.) The most dramatic and rapid gains have occurred in East Asia, where life expectancy at birth increased from less than 45 years in 1950 to more than 74 years today.
These improvements are part of a major transition in human health spreading around the globe at different rates and along different pathways. This transition encompasses a broad set of changes that include a decline from high to low fertility; a steady increase in life expectancy at birth and at older ages; and a shift in the leading causes of death and illness from infectious and parasitic diseases to noncommunicable diseases and chronicconditions. In early nonindustrial societies, the risk of death was high at every age, and only a small proportion of people reached old age. In modern soieties, most people live past middle age, and deaths are highly concentrated at older ages.
The victories against infectious and parasitic diseases are a triumph for public health projects of the 20th century, which immunized millions of people against smallpox, polio, and major childhood killers like measles. Even earlier, better living standards, especially more nutritious diets and cleaner drinking water, began to reduce serious infections and prevent deaths among children. More children were surviving their vulnerable early years and reaching adulthood. In fact, more than 60 percent of the improvement in female life expectancy at birth in developed countries between 1850 and 1900 occurred because more children were living to age 15, not because more adults were reaching old age. It wasn’t until the 20th century that mortality rates began to decline within the older ages. Research for more recent periods shows a surprising and continuing improvement in life expectancy among those aged 80 or above.
The progressive increase in survival in these oldest age groups was not anticipated by demographers, and it raises questions about how high the average life expectancy can realistically rise and about the potential length of the human lifespan. While some experts assume that life expectancy must be approaching an upper limit, data on life expectancies between 1840 and 2007 show a steady increase averaging about three months of life per year. The country with the highest average life expectancy has varied over time (Figure 4). In 1840 it was Sweden and today it is Japan—but the pattern is strikingly similar. So far there is little evidence that life expectancy has stopped rising even in Japan.
Figure 4. Female Life Expectancy in Developed Countries: 1840-2009
Source: Highest reported life expectancy for the years 1840 to 2000 from online supplementary material to Oeppen J, Vaupel JW. Broken limits to life expectancy. Science 2002; 296:1029-1031. All other data points from the Human Mortality Database (http://www.mortality.orghttp://www.nia.nih.gov/sites/all/modules/extlink/extlink.png) 100% 50% no-repeat;">) provided by Roland Rau (University of Rostock). Additional discussion can be found in Christensen K, Doblhammer G, Rau R, Vaupel JW. Aging populations: The challenges ahead. The Lancet 2009; 374/9696:1196-1208.
The rising life expectancy within the older population itself is increasing the number and proportion of people at very old ages. The “oldest old” (people aged 85 or older) constitute 8 percent of the world’s 65-and-over population: 12 percent in more developed countries and 6 percent in less developed countries. In many countries, the oldest old are now the fastest growing part of the total population. On a global level, the 85-and-over population is projected to increase 351 percent between 2010 and 2050, compared to a 188 percent increase for the population aged 65 or older and a 22 percent increase for the population under age 65 (Figure 5).
The global number of centenarians is projected to increase 10-fold between 2010 and 2050. In the mid-1990s, some researchers estimated that, over the course of human history, the odds of living from birth to age 100 may have risen from 1 in 20,000,000 to 1 in 50 for females in low- mortality nations such as Japan and Sweden. This group’s longevity may increase even faster than current projections assume—previous population projections often underestimated decreases in mortality rates among the oldest old.
Figure 5. Percentage Change in the World’s Population by Age: 2010-2050
Source: United Nations, World Population Prospects: The 2010 Revision. Available at: http://esa.un.org/unpd/wpphttp://www.nia.nih.gov/sites/all/modules/extlink/extlink.png) 100% 50% no-repeat;">.