I gotta say, although this is one of the most common things used to attack Islam with I always say you have to consider what was the norm back then.
Was this the norm or not? I don't really know but I am thinking yes.
Be it the norm or no, Mu claimed to be a man of God, a messenger sent and inspired by the almighty..
Now this good man chosen by God to be his messenger and last messenger to deliver the last message from the divine as he claimed,
simply drilled a 9 y/o kid!...how on Earth didn't his God scold him? Why couldn't Jibreel stop him and enlighten him;
why didn't his Allah simply tell him, "my man, your ppl do this all the time, it's wrong I can tell you, you do not do it, and must command Muslims to stop it, for it harms the kid both physically and mentally; and I know better dude..!!"
I know why, cuz Mu simply had no revelation whatsoever..!
'A'isha reported that Allah's Apostle married her when she was six years old, and he was taken to his house as a bride when she was nine, and her dolls were with her; and when he (the Holy Prophet) died she was eighteen years old. (Sahih Muslim, Book 008, Number 3311)
'A'isha reported that she used to PLAY WITH DOLLS in the presence of Allah's Messenger and when her playmates came to her they left (the house) because they felt shy of Allah's Messenger whereas Allah's Messenger sent them to her. (Sahih Muslim, Book 031, Number 5981)
If that was the norm, this never exonerates Mu. Applying same logic, (Muslims and apologetics) can also defend Mu and justify his actions, had he practiced female infanticide for example, which was perfectly fine at the time.!
Check that out, http://www.forwarduk.org.uk/key-issues/child-marriage