This book introduced me to the world of philosophy when I was just a wee pre-teen. And I think it's perfect for that exact audience bracket. 
 The novel itself is catered to the young adult - it does have a good plot and decent characters, but it's not Gaarder's best written work. 
 The book is really all about the philosophy, and some of the most important philosophers and their theories are packaged up nicely, concisely and comprehensibly in these pages. 
 Highly recommend it as a philosophical starter-pack for the young adult.