There are many ways a teenager can relate to this novel given the way someone who reads this story grows up. I am a young teacher (26) so I remember high school quite vividly. I grew up in an urban area and went to the high school known for a lot of crime. We had the gang wars, the booze, the drugs and I could really relate to how Bryon grew up. He rrealized it was all rote and began thinking for himself. The kids who are like Mark can read this story, see a lot of what he does in what they do and maybe have friends that ended up in jail or worse (like Charlie). On the flip side though, I now teach at a very small farm school that is the polar opposite of the type of high school I had attended. My students see a lot of the same types of problems in their lives as well.
One of the biggest things I noticed is in the middle of the book when Bryon makes the comment that Mom didn't know most of what they did as it was some cardinal rule that parents just aren't kept in the know, that is one of THE biggest assumptions that almost all teenagers make. Most teenagers think nothing will ever happen to them and their parents are oblivious to all they do. Common misconception as most parents DO know, they just choose to pick their battles.
S. E. Hinton was young when she wrote this and The Outsiders and having teenagers as the main characters helps to make it overwhelmingly realistic as well. The way the dialogue is set up, the problems they have to deal with and the way that they do are all characteristics of your typical teenager across the board.