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.
In any event, shorter life spans means marry earlier.
Also, in the old days, work meant physical labor while today, it often means intellectual labor. That means today, marriage, sex should be delayed, compared with the old days. However, the biology has not changed as much as the culture and that produces conflict.
Marriage for girls in their mid to late teens was acceptable. However, it was a much different world they lived it.