There's a bit of a misconception there.
The AVERAGE life expectancy was about thirties, but that is because of a much higher rate of infant mortality in those days than now.
If half of all people born in a given time die at or shortly after birth, and the other half live to be seventy years old, what is the average life expectancy?
It's not that a lot of people were only living to about thirty-something, and then dying at that age. It's that a lot of them wen't living very long past birth, pulling the average down.
Shakespeare himself only lived to 52, but both his parents lived to about seventy, and his children lived to aged ranging from 66 to 77, except for one who died at eleven.
Life was simpler, then, and one didn't need to know as much to be functional as an adult.
Consider, also, the Jewish bar Mitzvah tradition, which is based in a boy traditionally being considered to have become an adult at thirteen.
Eighteen as the age of majority is a fairly recent development, and I am beginning to think that in modern society, even that may be too young.