An oft-repeated factoid or ‘info nugget’ is that ancient and medieval people lived long lives, comparable even to today, after excluding child/infant mortality. Infant mortality skews the data so much that average life expectancy was otherwise similar to today.
Not quite so. Rather than the assurance of a long life conditional on surviving infancy or childhood, at best it’s a few extra decades, with survival beyond the age of 70, which could be considered elderly, being very uncommon. Yes, there is a bump in average life expectancy after the age of 10, but it’s not that much, maybe only 3-4 decades, even as recently as the 1800s.
From Wikipedia, Ancient Romans had a life expectancy of 35-40 years even after factoring out infant mortality, which is still pretty bad, or comparable to the life expectancy of someone born with a terminal condition like cystic fibrosis today:
When infant mortality is factored out [i.e. counting only the 67-75% who survived the first year], life expectancy is around 34–41 more years [i.e. expected to live to 35–42]. When child mortality is factored out [i.e. counting only the 55-65% who survived to age 5], life expectancy is around 40–45 [i.e. age 45–50]. The ~50% that reached age 10 could also expect to reach ~45-50; at 15 to ~48–54; at 40 to ~60, at 50 to ~64–68; at 60 to ~70–72; at 70 to ~76–77.