All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad (saw) is His Slave and Messenger. It was stated in As-Seerah Al-Halabiyyah that ʻAli was twenty-one years old when he married Faatimah and that she was fifteen years old.

Allaah knows best. (Source: islam_web) Classified in Merits and Virtues > Merits of the Prophet's Family