Oh I so didn't 'mis-spend' my youth! I was the swot of the class, I always did my homework, hardly ever misbehaved and never fell in with a bad crowd. I was in the school choir, acted in school plays and did well in exams. I wasn't allowed to mis-spend my youth - my parents didn't let me go out very often, not even to the cinema (I've been overcompensating on that one ever since!) and in fact didn't even let me listen to much pop music or wear fashionable clothes. With all the fuss these days about parents being paranoid about their children's safety I now realise my own parents were ahead of their time. I do wish that I had had more freedom as a child, though I've always been serious by nature and always enjoyed school and even homework and so was never really a candidate for 'mis-spending' my youth.
Things changed at University though I didn't mis-spend my time there I certainly had a good time. I joined lots of University societies, ones that were socially useful but also had a good social life (Student Community Action Group for example had me volunteering with dementia patients but also going to some brilliant parties, while Friends of the Earth were as famous for their pub nights as for their conservation work). I also worked hard but not too hard and passed the exams that mattered. I think I had the best possible time at University.
I only discovered live rock music in my 20s and clubbing in my 30s (thankfully a lot of people in the goth and alternative scene are older than the stereotypical clubber) though apart from the late nights and the sometimes controversial music, its not time mis-spent really, goths (in Edinburgh at least) are the nicest, most polite clubbers you can find and I can't drink alcohol when I'm dancing.
I'm glad I didn't 'mis-spend' my youth, though I do wish I had seen more films and listened to more music when I was a teenager, I feel I miss out on too many cultural references.