No body is sure about this.https://en.wikipedia.org/wiki/Aisha
A quote:
Aisha was six or seven years old when she was married to Muhammad with the marriage not being consummated until she had reached puberty at the age of nine or ten years old.
In civilized societies, there's a name for people who bang 9 year olds, and it's the opposite of holy. Some messenger of God!
I can say his marriage with Aicha was for reason she deliver his message to whole the world,most of Hadiths was recite by her.