Card has stated on many occasions that he never intended to write the book for a YA audience, but the fact that the characters were children opened the book to an unexpectedly young audience. The sequels to Ender's Game make that blatantly clear: they still grapple with pretty deep morality and philosophy, but without the packaging of children playing war games. It's easy to be distracted by the surface story in Ender's Game, because it IS a good story and fun, but there are a lot of undertones that I'm betting most kids miss when they read the book.