I was (and still somewhat am) into science fiction and fantasy around that age. Some of my favorites are as follows (most of these fall into the "fun reading" and not "classic" category:
A Wrinkle in Time, Dune, Hitchhiker's Guide (and sequels), Phillip Pullman's "His Dark Materials" trilogy (The Golden Compass, The Subtle Knife, and The Amber Spyglass), also some Michael Crighton books: Sphere, the Andromeda Strain, the Terminal Man, Jurassic Park, Prey.
If you haven't already read the following when you were younger, I'd still recommend anything by Roald Dahl (Charlie and the Chocolate Factory, The Witches, Danny the Champion of the World).
John Bellairs also had some good mystery/horror (The House with the Clock in its Walls, The Figure in the Shadows). More on the classic side, check out the works of Edgar Allen Poe. Also, The Phantom Tollbooth is a good one. Oh yeah, and I still like the Harry Potter books (nuff said!) even though I'm 26 /ubbthreads/images/graemlins/grin.gif.