Monogatari no Naka no Hito

After living hundreds of years as a recluse, Rihido a young man with eternal youth decides to join the ouside world again! He comes upon a daughter of a lord, Mycenae, being chased in the woods and helps her. He befriends her and finds out that a school of magic exists and decides he wants to become a student too. He changes himself into a teenager and uses any means to becomes a student!!
(Source: Alpha Manga)