I was listening to a debate between a Christian and a Muslim, and the Christian attacked the legitimacy of Mohammad's prophethood on the basis that he didn't really seem very moral, for reasons including the fact that he slept with a nine-year-old girl. In his rebuttal, the Muslim debater said that the Christian is being hypocritical because when they debate atheists the atheist will say that human society is sufficient for moral values, but Christians know that without God as the foundation of morality, human morals that are just a matter of convention are worthless (that's the Muslim's opinion, but of course I disagree with him about that).
So, the Muslim says that if the Christian agrees that moral values come from God, then show him where it says in the Bible what the minimum age of consent is, and if he can't do that (the Christian admitted he couldn't), then the only way he can say Mohammad was wrong to sleep with a nine-year-old is to appeal to an atheistic sense of morality, based purely on human convention.
As an atheist, I agree with him (just on that particular point, of course), but I wanted to you as a Christian the chance to take up the challenge and see if you can validate any minimum age of consent from a purely Scriptural basis.