Should children's books be content rated?

"What content-rating would you give the Hunger Games book?" my friend asked me.

"Probably the same as the movie.  Thirteen and up, for most kids." I said.

"Why don't they put ratings on books anyway? They do on music, TV, video games, and movies but children's books are so inconsistent. I feel like I have to screen every book my eleven year old picks up."

I couldn't answer my friend except to say that children's books target a broad spectrum of ages, emotional maturities, and reading levels making any rating system difficult if not impossible to apply. Not to mention that everybody seems to have their own opinion of what is appropriate and inappropriate for children.

If I were to draw a spectrum of children's books it might look like this:

Depending on the bookseller, the ages might be different on each of these sections but not by much.

So, what determines where a book falls on the spectrum? It comes down to what ages the book is accessible too, i.e. most of that age group would understand the book and find it interesting. Usually the age of the main character is a good indicator because if the character is having realistic, age appropriate experiences, the book will naturally fall into place on the spectrum. Vocabulary also tends to increase as you move left as does the complexity of the plot and characters. Some people say that MG books have main characters that are more internally focused where YA characters are externally focused but it's not hard to come up with exceptions to that rule.

So why do I think it would be almost impossible to rate books like movies? Children develop at different rates and have difficulty with varying aspects of life. A twelve year old (sixth grader) might not be able to read Harry Potter before bed without having nightmares but could read Wintergirls and have a deep discussion on the real threat of anorexia. For other kids it would be the opposite. Some people are sensitive to any sexual content. (I believe this was why Scholastic dropped Breaking Dawn). Others don't like violence, drug use, or swearing. All of these become more prevalent as you move left on the spectrum but it really depends on the book. 

Also, I think there's a vast difference between what we read and what we see. When we read, our minds must produce the picture from the words and if you don't have the life experience to produce that picture accurately you won't "see" it. For example, a young person who reads Breaking Dawn might have a very different picture of what happens in the bedroom scene than an adult. That's very different from TV, video, or movies where the picture is painted for you.

What do you think?  Should children's books be content-rated like movies or TV shows? Do you think there are topics, words, or experiences that simply don't belong in any children's books?