Legal maybe, moral no. I don't recall any of my American heroes marrying anyone in their teens. Most of them made their start in the world, became successful and then married someone their own age. And only one wife.
Not only that, but a person could be King of England or Pharoah of Egypt at an early age. This was usually followed by being a former King or Pharoah, still at an early age.
People reproduced as best they could and marriage frequently had less to do with sex than it did with raw survival.
Should you have early Jewish-Americans in the line, you will immediately notice they married their cousins, and at early ages, there frequently being no other persons to marry. Or, if you have Mennonites, you will find the same thing.
Then, there are the American Indians - lucky to find a formal marriage there, even if there are records.
Do not judge the past, particularly the ancient past, by your own modern standards which arise out of a condition of wealth and leasure. In olden times farms needed worked, fields ploughed, cows milked, the elderly in the family needed attendants, and the only way you got those folks was to "grow your own".
Shooter 2.5, also consider that those in America up to a hundred years ago that were marrying young were not the founders of a religion. They were probably uneducated farmers and laborers.
YOu need to read more. Try the life of "Ol' Hickory," Andrew Jackson for example.
He IS a "hero", but he had his ways...
(N.B.: Being played in film by Charlton Heston is a sure way to boost one's stock, though).
You probably need to rephrase that as "early teens" or pre-teens. Of course Muhammad's wife became a teen in her third year of marriage.