Check out this article in the Seattle Post Intelligencer about teens and YA fiction. Cecelia Goodnow takes a look at teens and their buying (and checking-out) habits and the new popularity of Young Adult fiction.
In the article, Goodnow writes, "As for which came first--the surge in quality or the receptive audience--no one is entirely sure." As a former teen, albeit 20 years ago, I believe the market was always there, as was the quality. The difference is in the quantity of quality. While 3-4 excellent YA books were published per year 20 years ago, now there are hundreds of worthy titles available for teens each and every year. It is a golden age.