I've been asked a few times about censoring a child's reading and I struggle with this answer.Truth be told, we didn't censor our daughter's reading growing up. What she read was up to her. However, I do understand this may not be appropriate for every child. I also hope I do not draw the ire of some people when I admit to our parenting style.
My daughter was a very advanced reader who started reading at a young age. This proved difficult in providing her with suitable reading material. The books in her age category weren't challenging enough so I had to begin to let her read above her level. I believe that if this is handled responsibly, it can be a good thing.
If you have an advanced reader, encourage them to continue reading, to progress ahead. Don't let them stagnate because you don't feel there are viable options for them. See where their interest takes them.
The one thing I found helpful was to always read the same books as she did. That way I would know the subject matter she was reading and we could discuss it in depth. It is a commitment of time on your part, but well worth the rewards!
The funny thing is with this lack of censorship came a whole new enlightenment: self-censorship. My daughter would look at a book, read the jacket, and say, "I don't think this one is appropriate for me." It followed suit with all aspects of her life. She was suddenly able to discern what was suitable and responsible for someone her age and what was not. This awareness, and degree of maturity, made parenting so much easier--not that I had any problem laying down the law when needed. However, 99% of the time we were able to keep an open dialog and she would listen with a keen mind as to why we would or would not allow something.
I truly attribute part of this personal growth to the freedom she had growing up and learning to censor herself. Again, I do not think or promote this for every child as every child is different. However, if your child is bored with their current reading level, encourage them to seek out more challenging books. Engage in those books with them. Together you will help build not only their literary accomplishments, but their maturity, and self-awareness as well.
Know your child. Know where they belong, and what they can and cannot handle, then allow them the freedom to responsibly explore...