Now you see girls playing with dolls till 16. You didn't had dolls back then and people were reaching puberty pretty fast. Helping the parents with harvest and such. Or even go to battles.
And the parents and the girl itself agreed to marry with the prophet.
So whatever you have to say, the law, your opinion about it, i don't care as long as the parents agree and the child itself agrees then there is nothing wrong with it.
Hundreds of people where there at the marriage of the prophet and no one has judged it and no one has disagreed, none of them.
So who are you to judge this 1400 years later?