For all we know, he could've been comatose for a while or something.

My headcanon says he was, because you know what? Going in there and killing the king of the Earth Dragons when you're seventeen(ish) years old is a big deal. Not a cakewalk. (Yeah, it is in terms of gameplay, but eh.)

Also, note the constrast between the FE11 endgame and the FE3 "grab throne, get glomped by Tiki, have a heart-to-heart with Caeda" ending. Well, he's an adult by then. And the dragon-killing thing is old hat by then, too.