Muslim scholars differ about the actual age of A'isha at the time of her marriage to the Prophet Muhammed. While a number of authentic reports mention she was nine, it is agreed she had reached puberty before consummation and Islamic law regards her as an adult. The prophet married Aisha as a means to cement his relationship with his close friend Abu Bakr.
Whatever her age, all scholars agree that she had reached puberty at the time of her marriage, since it is prohibited in Islam to have marital relations prior to that time.
It is also important to note that any discussion about age and marriage needs to be understood in the context of the culture, time and place - judging one set of civilisational values by another set makes little sense, as either set of values would be out of place in the other's context.