[dredmorbius]

Measures of life expectency are misleading in this regard.

Much of low life expectency comes from very high infant mortality -- deaths to age 5 or 10. Once you've passed that threshold, your odds of survival increase markedly. Perhaps slightly lower than today's first-world standards, but hale-and-hearty at 15 doesn't mean you're going to fall over dead at 25.

You also want to look at life expectency at any given age. And I distantly recall that the number of (I can't remember specifically which) Greeks or Romans surviving to age 80+ compared favourably to modern times.