Actually
even amongst societies which encouraged young marriage Europe in the medieval period for example, accepted medical knowledge realised that they weren't yet ready to have children regardless of sexual maturity. Just because your period has started doesn't mean your body is equipped physically for the birth of a child, you're still growing yourself and amongst other things as BGB pointed out your pelvis isn't equipped for it.

That's not even to bring in the obvious about mental and emotional maturity and the damage it can do to young girls
( Amberl was stripey and dominated Europe, Wed 3 Nov 2010, 17:55, Reply)