Paying the price
When I was a 'yoof', I too was guilty of listening to music with headphones way too loud, although not on public transport because back then portable CD players were way too expensive and I never really felt the desire for a cassette walkman.
Now I'm in my early 30s, I can definitely tell my hearing is on the way out. It's not such a big deal right now, although my ability to isolate individual sounds in a noisy environment is alarmingly bad (I'm useless at conversations in the pub) and the high-end frequency response of my left ear is somewhat worse than my right ear. I expect by the time I'm in my 50s, I might need a hearing aid.
Kids now listen to loud music way more than I did. Antisocial implications aside, I think we're going to have an awful lot of very deaf 40 year old men and women walking around in 20-25 years time. Thinking ahead, maybe I should consider developing a hearing-aid app for the iPhone....