Read Kenja no Mago Manga:
A young man who had surely died in an accident, was reborn in another world as a baby! After that, he was picked up by the patriot hero "Sage" Merlin Wolford and was given the name Shin. He was raised as a grandson by Merlin and soaked up Merlin's teachings earning him some shocking power however, when he became 15 his grandfather Merlin said: "I forgot to teach him common sense".