When I was that age I was reading fantasy and science fiction. I honestly forget what I was reading exactly, just that I distinctly remember at age 12 trying to write fantasy stories. Now that's some memories that need to be dug up. I easily can bring to mind the non-fiction stuff - things like the K&R C manual, the X Window System manual, the Amiga Rom Kernel Manuals. Some odd stuff about mandelbrot sets and chaos theory maybe. It blurs into what I definitely remember having been in my early teens, so I know my memory of the earlier stuff is vague.