I don't think that has anything to do with it MT, because, while he married her at 9, he didn't consummate the marriage until she had come of age (which at the time was puberty, which made sense since everyone died at 35 ;P )

I think it has more to do with protecting a girls "virtue" and "honour" before she can get involved in premarital relations and with the idea that the younger a girl is, the more child bearing years she has and the easier she is to "train."