I am looking for this manga. In this manga, as far as I recall, the main character kills the demon lord and finds his daughter. He then saves her (I guess) and somehow becomes a child again and then the demon lord's daughter, who is now a grown up, takes care of him. She also sends him to a magic school where she is the teacher.
After reincarnating in a different world, the great sage Ephthal spent all of his life researching magic. However, he who had aimed to reach the summit of magic, knew of the limits of his talents and fell into despair, thus closing the curtain to his lifetime. However, 400 years later, he reincarnated for the second time and enrolled in a magic academy using the sorcery and knowledge he acquired from his previous life. As if to sneer at the state of magic that had been in decline for the past 400 years, he easily invokes legendary class magic! With overwhelming power, the entire world will kneel before him.
The main character, Ephthal, was one of four sages who helped the hero to defeat the Demon King. In the aftermath of the battle, he also discovered the Demon King's orphaned baby daughter, Merlin, and adopted her as his disciple.
He eventually dies of old age and is reincarnated 400 years later as boy who's one of his own descendants. He then meets a much older Merlin, who -- after recognising him as her former guardian/teacher -- adopts him and enrols him into a magic academy, where she's the head teacher.