6-sai no Kenja wa Hikage no Michi wo Ayumitai

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.