FYI, even in the paleolithic era, if you lived past 15, the life expectancy was 54. [1]

Using average lifespan in your argument portrays a world where no-one lives to their 50s. When in actuality, most adults died in their 50s (but there was a very high level of child mortality which brings down the average).

[1] http://en.wikipedia.org/wiki/Life_expectancy