The reason the AVERAGE life expectancy was only 20, was that childhood mortality rates were so high combined and that injuries (like a cut or broken leg) were fatal. Cavemen didn’t just drop dead at 20. If you made it to adulthood as a caveman, the odds are that you would live a long time.
It’s the same reason the AVERAGE life expectancy in 1900 was only 40. The child mortality rates were high, which skewed the average.
The Bible says a man’s lifespan is 70 years (Psalms 90:10), yet the AVERAGE lifespan at the time was only 30. Again, people weren’t just dropping over dead at 30. It was the high child mortality rate that skewed the AVERAGE life span numbers. If you made it to adulthood, you had a good chance of living a long life.
Even if you take childhood mortality into account, during paleontological times, most people still didn’t live into their z0’s or 80’s.
You were considered pretty old if you made it to 60. Just look at the primitive tribes today to see that this is true.
I tend to follow the Primal diet[Paleo with dairy]. Also, the lifespan of man, or max now, is 120. As in Genesis 6:3.