*Ahem* I fairly disappointed that some comments actually sound like they came from DA users. Oh well, we can't complain about something that already happened. First of all, age doesn't mean anything. So quit bitching about it. It's no excuse for something as trivial as art skill. You can improve it whether your a child or a senior citizen. So I'll just get on with my crit.
The first notable thing about this picture is that obvious tilt you claim that you can't see. It's right there. You can't possibly miss it. Considering some colleagues already pointed it out with pictures. Firstly, the anatomy on this pic looks absolutely terrible. makes me sad on how this boy right here lives his everyday life. His head is disproportionate with his body size. His' neck is too thick and long and his legs looks like he has Polio. Especially the left leg, his' knees looks like it can't even connect the two halves of the leg. Poor fellow. I suggest you look at anatomy tutorials and/or books to improve your skill in this area. Because right now your anatomy looks utterly dreadful. Good thing you're only 15, that means you'd have a lot of time to refine it.
I also want to mention the mediocre shading, it doesn't make a bit of sense because I can't tell where the light in your drawing is coming from. Learn how to shade properly. I mean look at it, it looks odd. Also try adding multiple layers of shading. It looks bland and unappealing with one layer. And look at that hair! I mean they look like something a sock puppet would have for hair. I looks like yarn. That's something I don't see everyday.The sunglasses also strange.One lens is bigger than other. Plus why didn't you put any shine to the glasses? The heaphones also look unnatural. I don't understand how the hell that chair maintains to stand. It looks ready to fall down. Weird design I might add.
I'll end it here, because you'll probably fall asleep if I go any further. Or rage for that matter. But last thing I'll say about your pic is that "I <3 NG" looks damn unnecessary. Anyway, you'll definitely have a great chance to turn into a great artist someday. Just bear in mind you can never achieve this in a short time. Keep practicing, don't give up. And more importantly never let your ego take over no matter how good you become. Otherwise you'll wind up in an unwanted situation. Last question, who the hell is that deformed guy I've been critiquing? Oh, and always remember to stay away from DA. For obvious reasons.
Harsh crits for everyone!!!