As someone who has had mild hearing loss (I'm not even 30 yet!), I can speak for the bad effects of listening to loud music on headphones.
In a completely silent environment, you should gradually increase your volume from zero until you are able to distinctly hear all parts of a song (the vocals, the percussions, the bass line etc). Keep note to NOT increase the music volume more than 20-40% of this volume level.
From what I believe, the ability to not hear external noises should be perceived in the right context - You can be unable to hear external noises either because your music is too loud or because your headphones is external-sound insulating or noise-cancelling.
If your case is the latter, you're good - just remember to remove your headphones off your ears for a couple of times every hour to aerate your ears and reduce infection rates (and also to give your eardrums time to regain some of their strength).
If it's the former, then you need to reduce your volume to the lowest suitable volume.
A good rule-of-thumb is '60 minutes of music at 60% of your MP3 player's maximum volume'
Also, it's advisable to shift to large old-school headphones as compared to earphones and in-ear earphones. Happy listening!