It depends whether they are "nice" young people, behaving in a civilised manner, or "not nice" young people, behaving badly. I can't think of a word for the "nice" ones. Except, maybe, "typical teenagers". But the "not nice" ones are usually referred to as.....louts, layabouts, trouble-waiting-to-happen, little chavs......plus a few more vulgar terms which my wife won't let me write.

I guess, that is actually "Hanging out". It means spending time together with friends for fun or just to kill the time.