The prediction is not surprising. The set of people alive at age N has size Sn. Some of them die. Some survive and form a new set of people alive at age N+1 wi

The prediction is not surprising. The set of people alive at age N has size Sn. Some of them die. Some survive and form a new set of people alive at age N+1 with size Sn+1. Sn+1 <= Sn. If the expected number of remaining years at age N was Y, death removed all the people that actually lived only 1 year or less, so the expected number of remaining years for people of age N+1 must be greater than Y. Life expectancy at birth is always lower than at any other age. Check https://en.wikipedia.org/wiki/Demography_of_the_Roman_Empire https://en.wikipedia.org/wiki/Demography_of_the_Roman_Empire > When the high infant mortality rate is factored in (life expectancy at birth) inhabitants of the Roman Empire had a life expectancy at birth of about 22–33 years > The 46-49% that survived to their mid-teens could, on average, expect to reach around 48–54