A long time ago I posted somewhere about this very idea. ( I think it was here when Rabbit took over and changed some rules about posting stories containing underage characters.) I'm not going to do the research again, but my recollection is that here in the US there was a very large variation between states regarding age for marriage. In at least one state, I believe, a girl of 14 could marry without her parents consent, and in many states she could marry with their consent. In other states women under 21 had to have their parents consent. There was even one country, as I recall, which allowed girls as young as 9 years old to be given away in marriage (I'm not certain, but I think it was Afghanistan under the Taliban.)