Kenja no Mago

Thanks to various magic, the human race has been saved from calamity many, many times. Retaining his memories from his previous life, a boy, Shin, was picked up by an old man known to the world as the "Magi." Shin was brought up by the Magi, who had retired to a secular place, as his own grandson. With the memories of his past life, he absorbed the Magi's craft and grew to be able to develop his own magic. Then, the grandfather told him he has to become independent when he reaches the age of 15.