What is the wisdom behind the Prophet's, sallallaahu ‘alayhi wa sallam, marriage to ‘A’ishah when she was nine years old?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

In his book Ar-Raheeq Al-Makhtoom, Safiyy Ar-Rahmaan Al-Mubarakfoori said:

The Prophet's desire to be related to Abu Bakr and ‘Umar through marriage to ‘A’ishah and Hafsah, marrying his daughter Fatimah to ‘Ali ibn Abi Talib, and marrying his daughter Ruqayyah and then his daughter Umm Kulthoom to ‘Uthman ibn ‘Affan indicate that he aimed, through this, to reinforce bonds with those four men, who are known to have endured afflictions and offered sacrifices for Islam during the crises it went through; and, by the Will of Allah, it passed it safely.

To conclude, the Prophet's sallallaahu ‘alayhi wa sallam, marriage to ‘A’ishah was intended to strengthen the relationship with his friend and companion in hijrah (emigration), Abu Bakr . When he, sallallaahu ‘alayhi wa sallam, married her in Shawwal, during the eleventh year since the start of his mission, she was six. He consummated the marriage with her seven months after the hijrah, in Shawwal, and she was nine at that time. She was a virgin and was the only virgin that he, sallallaahu ‘alayhi wa sallam, married. He delayed the consummation of marriage until she became nine, the age at which a girl in this region, during that era, would usually attain puberty, menstruate, and be fit for sexual intercourse and procreation.

Allah knows best.