There was no Nikah or marriage ceremony performed between the blessed Prophet (sallalahu ‘alaihi wa sallam) and Aisha (radiallahu ‘anha) here on earth. Allah Himself married them. This means that Allah ordained that she be his wife. He could have done that even before she was born.

Would you any one else have a problem with Him ordaining that someone be wife of a man even before He Created her?

The marriage could not be consumated until she had reached the age of puberty – i.e., when a girl becomes a woman in the biological sense of the word. It was at that age that she went to live with her husband; and it was at that age that the angel came to Mariam and by Divine decree she became pregnant.

Do you have a problem with the Divine decision that a woman can become a wife, and can become pregnant when she reaches the age of puberty?