Hearing loss

No on headphones, pumping music directly into your ears, not a good idea. I see people with ear buds turned up high so they can hear over the street noise.

Yes depends on how loud you play your music even when you use your speakers. Loud like a rock concert. Would also cause hearing loss.

Age is the biggest factor.

Yes, you can mistreat your ears, but age comes for us all.

I actually find that using noise blocking IEM's reduces how loudly I play music, by the way.  Of course you always have the club kid in the subway playing music so loud everyone can hear it, but for everyone else, noise blocking is the way to go. :)

They might have to rename this place Audio-Gone.