It is important to note that life expectancy is an average. In many cultures, particularly before modern medicine was widely available, the combination of high infant mortality and deaths in young adulthood from accidents, epidemics, plagues, wars, and childbirth, significantly lowers the overall life expectancy. But for someone who survived past these early hazards, living into their sixties or seventies would not be uncommon. For example, a society with a life expectancy of 40 may have very few people dying at age 40: most will die before 30 years of age or after 55.

In countries with high infant mortality rates, life expectancy at birth is highly sensitive to the rate of death in the first few years of life. Because of this sensitivity to infant mortality, simple life expectancy at age zero can be subject to gross misinterpretation, leading one to believe that a population with a low overall life expectancy will necessarily have a small proportion of older people.

For example, in a hypothetical stationary population in which half the population dies before the age of five, but everybody else dies at exactly 70 years old, the life expectancy at age zero will be about 37 years, while about 25% of the population will be between the ages of 50 and 70. Another measure such as life expectancy at age 5 can be used to exclude the effect of infant mortality to provide a simple measure of overall mortality rates other than in early childhood - in the hypothetical population above, life expectancy at age 5 would be another 64 years. Aggregate population measures such as the proportion of the population in various age classes should also be used alongside individual-based measures like formal life expectancy when analyzing population structure and dynamics.

It is true that life expectancy in the Middle Ages and earlier was low; however, one should not infer that people usually died around the age of 30. In fact, the low life expectancy is an average very strongly influenced by high infant mortality, and the life expectancy of people who lived to adulthood was much higher. A 21-year-old man in medieval England, for example, could by one estimate expect to live to the age of 64.