I would have been starting Eye of the World at age 11, if I remember right. Around that time I had begun to enjoy basically adult fantasy/SF, though EotW was a considerable challenge in terms of time investment and comprehending plot subtext. That book changed the kind of books I could, and wanted, to read.

Prior to that, I think the best example of useful books would be the Narnia series, which I was glad to see on this list though I probably ate them up at about the age Tom suggested. I probably read The Hobbit sometime after that. Oh, and Shannara series! I almost forgot about that shit. Those were the kind of books that helped me out of childish reading, like Goosebumps or whatever. Man, I totally forgot about Sword of Shannara. I was obsessed with that series, which was totally lame in retrospect, but now I'm tempted to say that they really launched me into reading more formidable stuff.

This is fun to reminisce about, actually.