One thing I’m always telling people is that you shouldn’t feel compelled to stay in your proscribed age group category. It’s important to read anything, everything that interests you and give something a shot even if it seems to be “too” old or young. Talk to your librarian or bookstore employees if you’re worried about encountering mature content, but don’t be afraid to experiment. Just because a book looks a lot thicker (or thinner!) than what you’re used to doesn’t mean it isn’t a worthwhile read.
I’ve separated reviews by age here to the best of my ability, but everyone needs to work at their own pace, so please remember that these are only guidelines!