At (approx.) what age are a woman’s uterine muscles strong enoough for to have a vaginal birth?
The earliest reported natural birth was from a six year old.
Cite
At the age at which she is capable of falling pregnant. The uterine muscles are smooth muscle, they are quite different from the skeletal muscles that most of us are familiar with.
The uterus doesn’t do much until you get pregnant. Then in 9 months it makes impressively massive gains in size and strength.
The many heightened risks for young, physically underdeveloped girls giving birth have little to do with their uterii and strength of contractions.
Nitpick: It’s “uteri”, with only one i. “Uterii” would be the plural of “uterius”, if there were such a word.