What was the actual age of Ayesha (AS) when married to Prophet Muhammad (PBUH)?
EDIT: The too young age (6, 9 etc) is criticized most by Non-Muslims.
According to other sources Ayesha's age was:
Aishah ( رضي لله عنھا ) was six years old when she married the Prophet (صلى الله عليه وسلم) and nine years old when he consummated the marriage with her. This is what is proven from the books of hadeeth and has been agreed upon by all the scholars of Islam.
Some of the narrations are as follows:
Some narrations state that Aishah’s ( رضي الله عنها ) age was six while others state that it was seven. Imam An-Nawawi (source) while commenting on this hadeeth in his Sharh of Saheeh Muslim stated that Ad-Dawoodee said:
In another narration,
There are many similar authentic hadeeth in Sunan Abe Dawood, Sunan An-Nasaaee, Sunan Ibn Majah, Musnad Ahmad ibn Hanbal, Sunan Al-Baihaqi Al-Kubra, Mustadrak Al-Haakim, Al-Mujam Al-Kabeer of At-Tabaraani which proves the above.
I would recommend you to read this treatise, which clears the doubts, arguments that people pose and misconceptions regarding marriage and age of Aishah ( رضي الله عنها ).
Conclusion. She accepted Islam when she was 4 in 1 Besat (بعثت) so that She was about 18 in 2 Hijri.
So that we can conclude she was between 18 and 20 on her wedding day.