The Prophet: Married Aisha at Age Six; Sex at Nine

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Here are a number of references to the fact that Muhammad married Aisha when she was six, and had sex with her when she was nine year old. For example:

Narrated Aisha:
The Prophet engaged me when I was a girl of six (years). We went to Medina and stayed at the home of Bani-al-Harith bin Khazraj. Then I got ill and my hair fell down. Later on my hair grew (again) and my mother, Um Ruman, came to me while I was playing in a swing with some of my girl friends. She called me, and I went to her, not knowing what she wanted to do to me. She caught me by the hand and made me stand at the door of the house. I was breathless then, and when my breathing became all right, she took some water and rubbed my face and head with it. Then she took me into the house. There in the house I saw some Ansari women who said, "Best wishes and Allah's Blessing and a good luck." Then she entrusted me to them and they prepared me (for the marriage). Unexpectedly Allah's Apostle came to me in the forenoon and my mother handed me over to him, and at that time I was a girl of nine years of age. (Sahih al-Bukhari, Volume 5, Book 58, Number 234)

Narrated 'Aisha:
that the Prophet married her when she was six years old and he consummated his marriage when she was nine years old, and then she remained with him for nine years (i.e., till his death). (Sahih al-Bukhari, Volume 7, Book 62, Number 64; see also Numbers 65 and 88)

From Answering-Islam.org