PC World writes one of the reasons why teens download so much illegal music could be because they don't have credit cards to sign up for legal music download services:
Industry leaders have dismissed these issues as the consequence of being in a transitional period. Still, one has to wonder whether they've forgotten the core audience they were trying to appeal to through the rise of these legitimate solutions. A February 2006 CRIA/Pollara study found that 13-17 year olds were both the most likely to pirate and the most likely to own a portable MP3 player. It is uniquely difficult for minors to obtain legitimate MP3s.
The biggest and most obvious hurdle for teenagers is the unlikelihood that they own a credit card. Without one, a teen is effectively blocked from purchasing MP3s legally at any online venue. As MP3s are virtually all exclusively purchased online, this creates a severe problem. An adult with a credit card could buy music in their stead and pass the tunes to them. However, that would technically violate the contract terms of many of these services.