I was about 9 or 10 when I was first interested in paganism and I started with Teen Witch, a gift from my aunt. I love that book and have read it cover to cover many times, it has tons of markings in it and the covers are all bent
I also liked Wicca: A Guide for the Solitary Practitioner by Scott Cunningham at that age. Not to toot my own horn, but I was a little ahead of my peers reading level wise so I'm not sure if that book is too mature but I thought it was readable and definitely held my attention.
I don't practice anymore but I still pick up the occasional book here or there because I love to learn about anything pagan/wiccan.