From wikipedia
Most Muslim scholars have accepted the tradition that Aisha was nine years old when the marriage was consummated. This has in turn led critics to denounce Muhammad for having sexual relations with a girl so young, which in modern times would be classified as child sexual abuse.
Evidence of age nine at consummation
These traditions are from the hadith collections of Bukhari (d. 870) and Muslim b. al-Hajjaj (d. 875). These two collections are regarded as the most authentic by Sunni Muslims.
* Sahih Muslim Book 008, Number 3310: 'Aisha (Allah be pleased with her) reported: Allah's Apostle (may peace be upon him) married me when I was six years old, and I was admitted to his house when I was nine years old.
* Sahih Bukhari Volume 7, Book 62, Number 88 Narrated 'Urwa: The Prophet wrote the (marriage contract) with 'Aisha while she was six years old and consummated his marriage with her while she was nine years old and she remained with him for nine years (i.e. till his death).
* Sahih Bukhari Volume 7, Book 62, Number 64 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 Bukhari 8:151, Narrated 'Aisha: "I used to play with the dolls in the presence of the Prophet , and my girl friends also used to play with me. When Allah's Apostle used to enter (my dwelling place) they used to hide themselves, but the Prophet would call them to join and play with me. (The playing with the dolls and similar images is forbidden, but it was allowed for 'Aisha at that time, as she was a little girl, not yet reached the age of puberty.) (Fateh-al-Bari page 143, Vol.13)
* Sahih Bukhari vol. 5, Book 58, Number 234 Narrated 'Aisha: The prophet engaged me when I was a girl of six. We went to Medina and stayed at the home of Harith Kharzraj. 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).
Other hadith in Bukhari repeat this information.