OK, so now that the truth has finally come out and OP is divulging his true intentions. And you've read the Quran, of course you have, and have suddenly become and expert. I'm not even a Muslim I just honestly feel the hate is unwarranted, I'm curious about the religion and its message. I'm really not going to debate with you, I've seen your question listed 500 times. I'm not an Islamic scholar or expert, I can't give you an answer for why marriage was legal at that age for the "entire nation". Not just for one guy. But if you honestly believe that marriage back then was for the reasons people get married now, you are solely mistaken. If you want to believe they had a sexual relationship at her young age, you can believe that. But clearly they did not, clearly she was not abused as they highly respected each other above and beyond until they day they died. Their relationship as history would show it is nothing short of respect, love and peace.
Now today its 2015, several places, including the state of Massachusetts in the United States, still allow girls as young as 12 to get married in "exceptional circumstances" with the consent of a judge.
Was what he did illegal at the time? No. Was what he did immoral and disgusting? In only a sinister and hateful perspective. Marriage back then was also mostly for protection. Fear of rape, murder and slavery was always around the corner. Being married to such an influential person provided much needed protection. For if you were a wife of the prophet, it would be less likely you'd be harmed.