I’m on the other end of the fence on this one. Knowledge shouldn’t be withheld due to arbitrary lines. If the parents aren’t tuned into what their children are reading or if the kid feels they must hide away what they’re reading, it’s indicative of more serious issues than books.
I say this with a kid who’s just starting to learn their letters and not at a point of reading on their own yet. If they want to pick up Ulysses or the silmarillion, I’m not stopping them. I will warn them they’ve chosen books that are very advanced and they will have more questions than answers while reading.