wouldn't most people in our society act quite different from how they act today if expected lifespan was 35-40 years instead of 75-85?
This is a bit of a myth. Very high childhood and teen mortality brought average lifespan down, but once you were 20 making it to 65 was not rare at all. In places like America where plenty of good food was affordable old age was fairly common. Take a walk through an old cemetery and see for yourself.