Knight, I find it hard to agree with some of your thoughts. While certainly very precocious youngsters and teenagers consume the Lord of the Rings series like soda pop, in terms of language, plot complexity and exposition it's most certainly written for adults. And while most American middle and high schools assign To Kill a Mockingbird to its reading curriculum (I read it in fifth grade), it most certainly is not a "children's book" even if its main characters are children. I sincerely doubt that Harper Lee was targeting the book toward young readers.
I think Catch-up is right. A book aimed at a certain age reflects its age-appropriate content. That said, these days defining "age appropriate" can be a quagmire. My wife works as a librarian in a middle school (grades 6-8). Along with the usual Harry Potter and other series, the library also has all of the Twilight books, the Hunger Games books and other "young adult" (whatever that means) series and books that often incorporate language and sexual themes and violence I certainly never had access to in my sixth grade library. Of course, the library also has the Lord of the Rings series and other "adult" books since many students read them at a young age.
I would agree that the very best books written for children and young adults are those that adults can appreciate. Pterry's Tiffany Aching series is a perfect example--I like some of these books much more than any of the Rincewind books. In contrast, the worst children's books are ones that drive adults crazy. I hope that I never have to read "Goodnight Moon" to any baby or toddler ever again.
Books intended for very tiny children are boring to most adults, but they may be very good for the children. A very great deal depends on the child. Having a larger vocabulary than most adults does not necessarily mean that a bright child will be able to understand an adult book. Even if they understand all the actions in the story, their responses will be from a child's experience level.
I would say that the best books for children and young adults are those that have a quality of writing that any intelligent, perceptive person can appreciate, and ideally would also have layers of meaning that would reveal more nuances to someone who had more life experience (including having read more books). The Annotations files for Sir Terry's books show how much more was embedded in his books if you happen to catch his allusions.