Not a teenager, and I don't own an iPod or some other brand of mp3 player (no need to, my phone does that just fine, heh), but I can certainly weigh in on the omnipresence of music. If I'm sitting in front of my computer, I have music playing, or I'm playing a game/video/something else that precludes music. When I'm in the car? I definitely have music playing, the only time I ever turn it off is during phone conversations (typically by my passengers, not me).
Outside the environs of my computer room/car though, you won't catch me ambling throughout life sporting a pair of headphones (though I keep a pair in my pocket all the time just in case), as I'm either going to be at work, in the middle of something that involves concentration/conversation, or I'm trying to be asleep.
But if it's a choice between silence/background noise that doesn't concern me/music, I'm going to be playing music (I'm typing this up while listening to Devin Townsend for instance). There's nothing wrong with wanting some form of constant mental stimulation, unless your using it to avoid human interaction.