Why don’t teens buy more downloads? One reason is that they can’t. Julien McArdle writing on PCWorld.ca pointed out this obvious but often overlooked fact: download

purchases require a credit card. Pre-paid gift cards provide an answer for some. But they can be hard to fine, cumbersome to use and seldom come in the exact denomination needed. Perhaps download kiosks in struggling record stores that allow exact cash purchases are an answer.
Interoperability between services and devices remains the greatest reason that teens and the rest of us have been slow to warm to pay downloads. And the lack of variable pricing which leads to over-pricing doesn’t do much to win the battle against the urge to grab a free track via P2P or from a friend even if it’s "illegal".