There was no "average" lifespan back then. It is a bimodal curve, one peak young, one peak old. "Life expectancy" is far more about the size of the two peaks than an average. In modern society the old peak very much dominates, but even then "life expectancy at birth" is still substantially influenced by the size of the two peaks. Much of the difference across the developed world actually comes down to what doctors classify as a live birth. (Big negative correlation between infant mortality and stillbirth.)