Check out this CNET article on digital music market trends.
Most interesting to note is that the market for buying music to own on your digital music player isn't teens but adults in their mid 20's to mid 40's.
Most of the younger generation have MP3 players - more than half the teens do, but they generally use their players to download music they already own on CD rather than buying new tunes from services like iTunes.
Oh, and funnilly enough Apple's iPods own nearly 80% of the US digital music player market. Of course that won't be the case in France much longer!