The problem with that image specifically is that he's way too tall for an anime 12 year old. He looks weird because he should be about 5 and 1/2 heads tall like in the anime. Of course height in real children will differ, though proportions will generally stay the same. My issue is even in the anime with the few shirtless scenes he has - his body is that of a muscular adult, unlike the rest of the show where is frame is smaller but still developed enough to have a good physique. It's a minor annoyance, but something to take note off.