The modal age of death for hunter gatherers that survived to adulthood ranges from 65 to 72 among the reasearched cultures.

Sure, a large proportion of them die at early childhood - "life expectancy" measures takes that into account by definition, as it measures the total average life expectancy; and if you get a 100 kids who live to 70 and 100 kids who die as infants, then the average life expectancy is 35.

Maybe you could present a paper? Because amongst most hunter gatherers researched the modal age is much lower...

M.Gurven & H.Kaplan Longevity Among Hunter-Gatherers: A Cross-Cultural Examination has a conclusion of "The average modal age of adult death for hunter-gatherers is 72 with a range of 68–78 years."

It's not about the ancient societies but is relevant because for this discussion the point is not history, but the effects of a hunter-gatherer diet.

http://www.anth.ucsb.edu/faculty/gurven/papers/GurvenKaplan2...

Sure. They also had neither vaccinations nor antibiotics, had no understanding of bacteria, etc. Take any modern teenager and stick him back in the stone age and I doubt he makes it to 40 even WITH modern knowledge.