Sahih al-Bukhari 3896—Khadijah died three years before the Prophet departed to Madina. He stayed there for two years or so and then he married Aisha when she was a girl of six years of age, and he consummated that marriage when she was nine years old.
Sahih al-Bukhari 5133—Aisha narrated that the Prophet married her when she was six years old, and he consummated her in marriage when she was nine years old. Then she remained with him for nine years (i.e. till his death).
Many Muslims in the world today live far better lives than their prophet lived. Not surprisingly, some of them are horrified when they hear that their highest moral example (Qur'an 33:21) had sex with a little girl. It is therefore becoming popular to manufacture weak arguments in an effort to conceal Aisha's age.
Recently, Jonathan Brown, an Islamic scholar who teaches at Georgetown University, was confronted by Adam Deen (of the Deen Institute), a Muslim who is apparently concerned about Aisha's young age. Adnan Rashid chimes in as well, and Brown defends the traditional age of Aisha.
Brown gives the best defense that can be given for Muhammad's marriage to Aisha, namely, that the world was different back then. But there are two massive problems with his response. First, consider Brown's "Goat Herder" defense. He asks us to picture a young seventh-century teenage boy, in the midst of a puberty-induced hormone rage, tending his goats. With nothing else to do, why wouldn't such a boy get married? Notice that even if we agree with this defense, it has little to do with Muhammad's marriage to Aisha. Muhammad was not a young boy in the throes of puberty. He was more than fifty years old. So the question isn't "Why would a young boy marry a young girl?" but "Why would a man whose life is nearing its end marry a prepubescent girl?" This problem is magnified by the fact that Muhammad's wives weren't allowed to remarry after his death (Qur'an 33:53), which left Aisha as a widow for the rest of her long life.
Second, when it comes to Muhammad, one can't appeal to cultural differences as a defense of obviously immoral actions. For Muhammad is the highest moral standard for Muslims of all times, not simply for the Muslims of his own time. Muhammad is the standard of correct behavior, which means that his marriage to Aisha was a pattern that may be followed even today. Thus, in defending Muhammad's sexual relationship with a prepubescent girl, a Georgetown University scholar just defended a Muslim's moral right to have sex with prepubescent girls.
Notice also the inconsistency of this "cultural" defense. Muslims tell us that one of the great things about Muhammad was that he challenged the immoral practices of his time, such as female infanticide. But wasn't female infanticide an accepted cultural practice for some groups during Muhammad's time? If so, why challenge the practice? It seems that Muslims want to use culture in whatever way seems convenient. If Muhammad changed an evil cultural practice, Muslims say, "You see! This proves that he was a prophet, since he challenged his culture instead of going along with it!" But if Muhammad engaged in an evil cultural practice, Muslims say, "But you can't condemn him for it, since it was accepted in that culture!"
For more on Muhammad's relationship with Aisha, see: