This only becomes a problem if we look at it through our cultural lens, which is unfair. From what I remember, the age of Aisha was around 6 when they married, and 9 when the marriage was consummated. Muhammad :saws1: was, I believe, around 50. This was a common custom at the time, and nobody thought ill of the marriage. If there was any sort of problem, then the enemies of Islam would surely have picked up on it and used it in their attacks, right?
From Wiki:
Child marriages such as this were relatively common in Bedouin societies at the time, and remain common in some modern societies worldwide.[14] British scholar Colin Turner suggests that such marriages were not seen as improper in historical context, and that individuals in such societies matured at an earlier age than in the modern West.[14] In modern times, however, the issue of Muhammad marrying and having sexual relations with a girl so young has been used to criticize him, particularly in societies where child sexual abuse and related issues are considered serious crimes.[14]