I met some 13-year-old kids online that are quite immature, but seem to have a good ear for metal. They listen to Opeth, My Dying Bride, Novembre, CoB, Slayer, Iron Maiden, Powerman 5000, Celtic Frost, Slayer, In Flames, Iced Earth, Edge of Sanity, Nevermore, Sinergy, etc...
I think that he is immature because he acts like he is and he doesn't respect anything soft. Even Led Zepplin is too soft for him. Go figure... He'll realize that there is a lot of non-metal music is good.
Also, I am 18 and I have liked Opeth since i was 16. My little brother, now 13, likes Opeth, but he would never admit it (he doesn't want to give into metal :/). I catch him listening to Opeth all the time, but denies his likings.