>>64449208
You can get pregnant around 12 years old, and people used to work from around the age of 7 onwards. This gives the girls a good 5 years of labor before it's time to start breeding, and the boys can work for 28 years before they keel over and die.

Hell, most people would become grandparents. A girl would give birth by 13, and then 12 years later she'd be only 25 when her child is pregnant. She'd have a whole 10 years left to enjoy her grandkids.