Bushmen live to be in their late 40's, and frequently into their 60's.
So as for the longevity issue... the average longevity for agricultural people before the Industrial Revolution was 30. This includes medieval Europe. 30.
Hunter Gatherer: 40-50 and with better health.
Of course AMERICANS and EUROPEANS and the JAPANESE are living longer because of fossil-fuel powered medical technology.
So while we may live LONGER than a hunter gatherer, our QUALITY OF LIVE is much less.
These are happy, content and playful people. Look at the average American. Not a happy picture.
So don't look at things so simply. "They live to be 20". They live to their 60's.
"AVERAGE LIFESPAN" includes infant mortality... which they have high levels of. But once you are the age of 13 you can expect to live to be around 40-60. Thx.