Manga by author Manimani Ononata

Douglas, once known in legend as the strongest magician.But now he is called old man, as his health slowly crumbles. Beyond just suffering from a mysterious disease that slowly, but permanently, saps his health if he uses magic, the hero's party forces him out and the adventurer's guild strips him of membership.Thus, in the middle of an aimless solitary journey, the ex-adventurer Douglas encounters a cursed girl, Ravi.

I have probably defeated the Demon King. The most powerful sage has been reborn as a 6-year old child named Edy. Although Edy inherited his previous life's memories and abilities, and possess more power than he previously had, he decides to lead an ordinary life. However, as he enjoys his peaceful life, he, as a boy with the soul of a powerful sage, once again gets involved with a world-saving mission under the name Darkness Ruler. Link : Artist's Twitter: https://twitter.com/cmcom345