A brief thought: Several agents, authors, and even a couple of editors whose blogs I read, have wondered aloud why Young Adult fiction is suddenly so popular. In fact, there is a debut author out there who signed a seven figure contract for a young adult series. That's almost unheard of these days, especially for a debut author, and especially with books that cost between 5%-15% less than normal books. (I think my numbers are correct.)
Here's my theory: E-books are becoming very popular, but publishers don't make as much money from e-books as they do from print. The numbers for authors are a bit fuzzier (agents can give you more solid numbers, and if any of you know them, please post them in the comments). The problem with e-books is that the devices used to read them are expensive. And the books themselves cost (typically) $9.99. Most mass market paperbacks cost between $6.99 and $8.99. At the end of the day, teenagers and children don't have enough money to buy e-readers, and since their books are cheaper in print anyway (in mass market they're about the same price regardless of the age they're marketed to), they can afford to buy more "analogue" books that they can keep on a shelf, lend out to friends, re-gift and re-read without worrying about losing a $100+ e-reader.
In short, young adults can afford to buy more printed books because they can't afford to buy e-readers.