Albert Barnes on Acts 7:23

Full forty years of age. This is not recorded in the Old Testament, but it is a constant tradition of the Jews that Moses was forty years of age when he undertook to deliver them. Thus it is said, "Moses lived in the palace of Pharaoh forty years; he was forty years in Midian; and he ministered to Israel forty years." (Kuinel.)

To visit, etc. Probably with a view of delivering them from their oppressive bondage. Comp. Acts 7:26.

(a) "And when he was" Ex 2:11