I think the way this was explained is that he only started out slower but has finally reached the teen years of HIS people which happens to match with the teen years of his friends.
I was going to ask that. I just finished book 2, and it is still not fully explained. I'm going to hypothesize that he will continue to age at the same rate as 'the black headed ones'™ throughout his teens, alongside his friends until his mid 20's as Jadith appeared to have.