I know plenty of 30+ year olds who aren't mature enough to make such a decision either. The age of 18 is just an arbitrary choice to plant a legal marker, because it's not practical to judge on a case by case basis.

Not that I'm condoning the behavior in this case....I'd think the intent of age of consent laws would be to protect those not yet able to protect themself with their own judgement - it should be obvious in this case, especially considering his age at the time, that taking a ~13 year old "wife" and proceeding to have sex with her when you had no intent to move there permanently was an incredibly wrong thing to do, by any moral compass.