In the "old days" of the US, I think kids got married very early, mostly because of the lack of medical care, they died very early too.
Also, a few hundred years ago, the average life span was about 30 or 40 which necessitated very early sex, marriage and child bearing.
While this is not the case now, a lot of human biology goes against behavior that is more favorable to today's reality of delaying sex and marriage until very much later.
Your figures are entirely wrong.
"A few hundred years ago the lifespan was 30 to 40 years"??? Your invention.
Marriage in the late teens and early twenties was common until recently, when free sex became culturally acceptable. There is nothing wrong with late teen/early twenties marriages, btw.
The only reason the averages say shorter lifespans (and your figures are way, way off) was becasue of higher infant mortality. Once a person made it through childhood, many made it into their 70s, and this is consistent throughout much of history.