Ok so any fan of the show Avatar knows that he is the last Airbender. Like ever. They were all killed by an imperialistic Fire Lord named Ozai. Well that's a heavy thing to deal with. Right? Being the sole survivor of an entire race. That's a lot for a 12 year old to handle. So it's my speculation that when Aang reaches 15-17 years of age he falls in with the emo crowd. I believe this is even what he may of looked like during that phase in his life.