It all depends on how I'm physically listing to the music (headphones, ear-buds, speakers, etc.), if I'm actively listening or passively listening to the music, the reason I'm listening to the music, and the particular song.
If I wanna get pumped up I listen to relatively fast and loud music. If I wanna relax, I listen to mellow music at a moderate volume. If I treat myself to some music while I sleep, I keep it quiet (for obvious reasons).
Headphones provide sharp and clear audio, so I don't need the audio too loud (a little less than moderate). I try to not use ear-buds because I love my ears and wish no harm upon them. If I use them, the volume is loud enough so I can hear clearly, but no more than halfway.
I am 21 and my ideal way of listening to music is through noise cancellation headphones at a medium volume so I can get the best audio out of the songs!
(p.s. I love music so I am obligated to be picky)