So it seems that humans evolved with a characteristic life span. Mortality rates in traditional populations are high during infancy, before decreasing sharply to remain constant till about 40 years, then mortality rises to peak at about 70. Many individuals remain healthy and vigorous right through their 60s or beyond, until senescence sets in, which is the physical decline where if one cause fails to kill, another will soon strike the mortal blow.
So what is the source of the myth that those in the past must have died young? One is to do with what we dig up. When ancient human remains are found, archaeologists and biological anthropologists examine the skeletons and attempt to estimate their sex, age, and general health. Markers of growth and development, such as tooth eruption, provide relatively accurate age estimates of children. With adults, however, estimates are often based on degeneration.
We are all able to instinctively label people as “young,” “middle-aged,” or “old” based on appearance and the situations in which we encounter them. Similarly, biological anthropologists use the skeleton rather than, say, hair and wrinkles. We term this “biological age” as our judgment is based on the physical (and mental) conditions that we see before us, which relate to the biological realities of that person. These will not always correlate with an accurate calendar age, as people are all, well, different. Their appearance and abilities will be related to their genetics, lifestyle, health, attitudes, activity, diet, wealth, and a multitude of other factors. These differences will accumulate as the years increase, meaning that once a person reaches middle age, the differences are too great to allow any one-size-fits-all accuracy in the determination of the calendar age, whether it is done by eye on a living person or by the peer-preferred method of skeletal aging. The result of this is that those older than middle age are frequently given an open-ended age estimation, like 40-plus or 50-plus years, meaning that they could be anywhere between 40 and 104, or thereabouts.
The very term average age at death also contributes to the myth. High infant mortality brings down the average at one end of the age spectrum, and open-ended categories such as 40-plus or 50-plus years keep it low at the other. We know that in 2015 the average life expectancy at birth ranged from 50 years in Sierra Leone to 84 years in Japan, and these differences are related to early deaths rather than differences in total life span. A better method of estimating life span is to look at life expectancy only at adulthood, which takes infant mortality out of the equation; however, the inability to estimate age beyond about 50 years still keeps the average lower than it should be.
Archaeologists’ age estimates, therefore, have been squeezed at both ends of the age spectrum, with the result that individuals who have lived their full life spans are rendered “invisible.” This means that we have been unable to fully understand societies in the distant past. In the literate past, functioning older individuals were mostly not treated much differently from the general adult population, but without archaeological identification of the invisible elderly, we cannot say whether this was the case in nonliterate societies.