"This date refers to the early days of Islam. For it is known that Aisha's sister Asma, who was born in 595, was 15 when she became a Muslim [11]. This indicates the year 610, when the Prophet started to receive the revelation and this then shows that Aisha was at least 5, 6 or 7 that day and that she was at least 17 or 18 when she married the Prophet in Medina."
http://www.islamicity.com/articles/articles.asp?ref=ic0811-3718