look at table: "Life expectancy variation over time" at:

http://en.wikipedia.org/wiki/Life_expectancy

At birth the average life expectancy is 28. But if you limit the count to population over 15, eliminating the infant and youth fatalities, it shakes out at 52 years.

Reference the "Life Table":
http://www.utexas.edu/depts/classics/documents/Life.html

As I read it a fortunate 2.2% of the population made it past 70.

I hadn't looked into it at any depth previously, not saying spending five minutes Googling passes for research, and gathered my points from what I remembered from reading about Roman life and noting the lifetimes of notable figures. It had always struck me that it is always assumed that ancient people died young whereas a lot of the famous ones seemed to live quite long.