I've learnt that these sort of topics always come around.
My mum always lectures my brothers about Zinaa and how it is a major sin. To be honest this is why I advocate early marriages. It saves a lot of people into sinning. But you equally have to make the guy responsible early on and earning from early on, so by the time his 21-22, his working and able to take care of a family (or at least him & his wife). From what I've seen a lot of guys can wait that long, even without being practicing (they must not have bad friends though). As for the practising ones, they wait as long as they get a wife.