Hearing loss

It would likely depend on what volume they're listening to them, wouldn't it?

If you live someplace where you can blast your speakers all the time, that will probably affect your hearing adversely, too.

I've been wearing hearing aids for about 15 years, but I'm 70 now.... 

Hearing aids, eyeglasses - it's all good! 

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. :)