If you're on your own and can't handle yourself, confronting them is often a recipe for disaster, as it's exactly what they're looking for. A couple of people around here have tried that before and got battered. Only one case went to court and the outcome was pathetic considering the damage done.
Some of these fifteen year olds are six feet tall and won't think twice about assaulting you if you offend or threaten them. If they're anything like the ones around this way, they've grown up in rough areas and been in lots of fights. This then puts you in a fight or flight situation and the former is likely to go very badly for most people, even if you "win". Even if you try to run away after confronting them, they will probably catch you.
I think different groups are being described in this thread. Some are just troublemakers nicking things from shops and just being a general nuisance, and others are looking to fight adults to show how tough they are.