It was a divine marriage i believe he had a dream in which an angel came to him and declared that Allah wants the prophet to get married to Aisha (RA) she was to be the mother of the believers.
We must comprehend that Hadith is by no means absolute only the qur'an is , hadith can contain human error (hence whys gem are known as being too weak or completely wrong as they contradict the qur'an) some scholars disagree with Aisha's age because studying the prophets siira (biography) they found that not long after their marriage Aisha participated in a battle (not sure which one) the minimum age allowed for males to participate in battles is 15 and for females its between 16-18 , kids are by no means allowed to participate and so they use that part of the biography to suggest that perhaps Aisha's age has been misrepresented , and no we muslims do not lie to cover our tracks whatever that means it just healthy skepticism.
Either way we muslims accept the wisdom of Allah , and if he willed Our prophet to marry Aisha to bring long term benefit to the believers than Allah is all wise we accept his wisdom , and if there is some sort of human error in the documentation of Aisha's age then again we accept that this can happen for only the qur'an is Infallible.
I hope this helps.