Why did Muhammad the last Prophet as per Koran, being a married man still felt the need to marry Ayesha at 9?

In the Hadith, it is said that Muhammad had many wives including Khadija, at one time, his Boss. But still he is said to have married Ayesha, a 9 years old girl who was more than 35 years younger than him. He is accused of being a child molester by the non-Muslims. What could be the truth? If he is not defensible, why the Muslims protect him and his past?