Short answer
The classical Sunni position is nine, because Aisha said so herself in reports found in both Sahih al-Bukhari and Sahih Muslim. The view that she was about nineteen is a modern argument built on estimates of other people's ages, and most scholars do not accept it.
The accepted Sunni account is that her nikah was at six and the marriage began at nine. No classical scholar of hadith or history held that she was nineteen; that view appeared in modern times.
The four schools agree on this.
Explanation
The case for nine is simple. Aisha described her own age, and the report comes through more than one student. Urwa ibn al-Zubayr narrated it from her, and al-Aswad narrated it too, with the extra detail that she was eighteen when the Prophet ﷺ died. Both are in Sahih Muslim. The great hadith critics accepted these chains.
The case for a later age usually runs like this. Some biographers say her older sister Asma was ten years older than Aisha and died in 73 AH aged about a hundred. Counting back, Aisha would have been around seventeen to nineteen at the Hijrah. Others add arguments about when she was born or when she accepted Islam.
Scholars answer that these are rough estimates. Ages like "about a hundred" and "ten years older" were often rounded, and they reach us in later biographical works without the strong chains of Bukhari and Muslim. A clear statement from Aisha about herself is stronger than a calculation from round numbers about someone else. Some critics also say that only Hisham ibn Urwa narrated it in old age; the narrations of al-Zuhri from Urwa and of al-Aswad from Aisha, both in Sahih Muslim, do not go through Hisham and answer that.
So the classical and mainstream answer stays nine. Muslims who meet the later view should know it exists, know why most scholars reject it, and stay calm. Respect for Aisha and for the Prophet ﷺ does not depend on winning an argument about numbers. See How old was Aisha when she married the Prophet? and her life.
The evidence
Sahih Muslim 1422
Sahih... تزوجني رسول الله صلى الله عليه وسلم لست سنين ...
"The Messenger of Allah married me when I was six."
Aisha's own words. Other chains of this hadith in Muslim come through al-Zuhri and through al-Aswad, not through Hisham ibn Urwa.
Sahih al-Bukhari 3896
Sahih... ونكح عائشة وهي بنت ست سنين ثم بنى بها وهي بنت تسع سنين ...
"He married Aisha when she was six, then began married life with her when she was nine."
Urwa's report, which also dates Khadijah's death to three years before the Hijrah.
Where the ruling is recorded
- Fath al-Bari (Ibn Hajar), Commentary on Kitab Manaqib al-Ansar, chapter on the Prophet's marriage to Aisha. Gathers the reports of her age and dates.
- al-Isaba fi Tamyiz al-Sahaba (Ibn Hajar), Entry on Asma bint Abi Bakr. The source of the 'ten years older' and 'about a hundred' estimates used in the modern argument.
- al-Bidaya wa al-Nihaya (Ibn Kathir), Events of the year 73 AH, death of Asma bint Abi Bakr. Records Asma's death and the age given for her.
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.