Short answer
Aisha herself said the marriage contract was made when she was six and she joined the Prophet's ﷺ household when she was nine. This is reported in Sahih al-Bukhari and Sahih Muslim, and it is the account of the classical Sunni scholars.
The mainstream Sunni account, taken from Aisha's own words in Bukhari and Muslim, is that her nikah (marriage contract) took place in Makkah when she was six, before the Hijrah, and that she moved into the Prophet's ﷺ home in Madinah at nine, after she had reached maturity.
The four schools agree on this.
Explanation
Aisha bint Abi Bakr narrated her own age. She said the Prophet ﷺ married her when she was six and the marriage began, when she moved to his home, when she was nine. Her nephew Urwa and other students passed this on, and al-Bukhari and Muslim both recorded it. Another narration in Muslim adds that she was eighteen when he passed away.
Two stages are meant here. The nikah was agreed in Makkah after the death of Khadijah. Aisha stayed in her father's home for about three years. The marriage itself began in Madinah, after the Hijrah, once she had reached maturity. Classical scholars treat physical maturity, not a number of years, as what matters for a marriage to begin.
People today often ask this question with worry. Some points help. Marriage customs and ages differed greatly between societies and centuries, and early marriage was normal in seventh century Arabia and in much of the world. The Prophet's ﷺ fiercest enemies attacked him on many things, but they never raised this marriage against him. Aisha speaks of her marriage with love and pride all her life, and she became one of the greatest scholars of the Companions.
Some modern writers argue that she was older, around eighteen or nineteen. We look at that view in Was Aisha 9 or 19? The Quran itself does not give her age: see Does the Quran say how old Aisha was? Her full life story is on Aishah bint Abi Bakr.
The evidence
Quran, Surah Al-Ahzaab 33:6
ٱلنَّبِىُّ أَوْلَىٰ بِٱلْمُؤْمِنِينَ مِنْ أَنفُسِهِمْ ۖ وَأَزْوَٰجُهُۥٓ أُمَّهَٰتُهُمْ ۗ وَأُو۟لُوا۟ ٱلْأَرْحَامِ بَعْضُهُمْ أَوْلَىٰ بِبَعْضٍۢ فِى كِتَٰبِ ٱللَّهِ مِنَ ٱلْمُؤْمِنِينَ وَٱلْمُهَٰجِرِينَ إِلَّآ أَن تَفْعَلُوٓا۟ إِلَىٰٓ أَوْلِيَآئِكُم مَّعْرُوفًۭا ۚ كَانَ ذَٰلِكَ فِى ٱلْكِتَٰبِ مَسْطُورًۭا ﴿٦﴾
The Prophet is more worthy of the believers than themselves, and his wives are [in the position of] their mothers. And those of [blood] relationship are more entitled [to inheritance] in the decree of Allah than the [other] believers and the emigrants, except that you may do to your close associates a kindness [through bequest]. That was in the Book inscribed.
Aisha, like all his wives, is honoured as a mother of the believers.
Sahih al-Bukhari 5133
Sahih... تزوجها وهي بنت ست سنين وأدخلت عليه وهي بنت تسع ...
"He married her when she was six, and she was brought to his home when she was nine."
The basic report of her age, from Aisha herself.
Sahih Muslim 1422
Sahih... تزوجني رسول الله صلى الله عليه وسلم لست سنين ...
"The Messenger of Allah married me when I was six."
Muslim's narration, which in one chain adds that she was eighteen when he died.
Where the ruling is recorded
- Fath al-Bari (Ibn Hajar), Commentary on Kitab al-Nikah, chapter on a man marrying off his young daughter. Explains the narrations of her age and the scholars' discussion of them.
- al-Isaba fi Tamyiz al-Sahaba (Ibn Hajar), Entry on Aisha bint Abi Bakr. Gives her birth, marriage and death as the biographers recorded them.
- al-Bidaya wa al-Nihaya (Ibn Kathir), Events of the first year after the Hijrah, the Prophet's marriage to Aisha. Places the start of the marriage in Madinah after the Hijrah.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.