The danger is not specifically from using closed-backed headphones, but of having them too
loud and using them or too long at a stretch. The same dangers are present when using
in-ears, open-backed headphones and high-resolution speakers (the latter because the very
low levels of distortion make them seem much quieter than they really are!)
There is currently nothing that can be done to restore hearing accuity once damaged --
at least not to a level that would allow skillful audio processing and balancing -- so the
only option is to look after your sense of hearing as carefully as you can, and that means
minimising your exposure to loud sounds. You can do that by reducing the volume through
sensible level settings and/or attenuating earplugs, and by reducing your exposure time by
taking regular breaks.
I'd much rather still be able to enjoy music and
converse easily in my dotage then appear 'cool' or 'hard' in my teens... which is possibly
why I was never deemed 'cool or hard' when I was a teenager!
Technical Editor, Sound On Sound