Basically, this isn't his first time. It's a constant cycle of him growing older until I think around 25, then he reverts back to a child WITHOUT memories. That's the kicker here; he somehow still remembers this time. Mio learned of this by visiting his father, his only living parent. His father is already so old and near death's door, but tells Mio that he hopes with her presence, something can change. Because even at his age, he wasn't able to find why this happens or how to stop it. Yet, somehow he still remembers his life and her.