Life expectancy is commonly defined as the number of additional years of life expected at a specified point, such as which of the following sets of ages?

• A. Birth, 45, 65
• B. 20, 40, 60
• C. 30, 50, 70
• D. 10, 30, 50

Answer: (A)

Life expectancy is the average number of additional years a person is expected to live at a given age, based on current mortality patterns. Because survival to a certain age changes the remaining risk of death, the remaining years differ by the age you start from. Using birth as a reference shows the overall expected lifespan from the start of life, while using ages like 45 and 65 shows how many more years a person who has already reached those ages can expect to live. These ages are commonly reported in public health to illustrate how longevity changes with aging, which is why the set including birth, 45, and 65 is the best example. Other age sets aren’t standard reference points for life expectancy, though life expectancy can technically be calculated at any age.