In contrast, only 19 percent of Swedes are under age 15, and 17 percent are ages 65 or older. To better gauge the life chances of individuals in a population and to better compare mortality conditions among countries, we must look at the average life expectancy. Both biological and social factors influence how long individuals live, and by extension, the average life expectancy within a population. In 1996, the average life expectancy at birth in the United States was 76 years. The lowest life expectancy estimates for the late 1990s were 34 years for Sierra Leone and 36 years for Malawi.
Life expectancy is a concept many people use but few understand.
As an infant born that year proceeds through life, mortality conditions will change. Individual lives may be cut short by war or a devastating plague of some new disease, or may be extended through fantastic medical advances. Life expectancy at birth in 1996 applies to no real group, not even to real people born in 1996. Like the TFR, life expectancy applies to a hypothetical group of people who pass through their imaginary lives subject to the death rates at each age in 1996. Age-specific death rates refer to the number of deaths of persons within a specific age group divided by the total number of people in that age group.