the story is he's lived all over the world, including scotland, and he's picked up a mixture of accents, hence the mixture of american, scottish and australian. and i think thats actually his accent!
but i agree, they could have got a 16 year old ot play a 16 year old, but then they wouldnt have been able to give him certain story lines hes going to play so it had to be someone older iygwim