The Lexile Framework for Reading is an internationally used measuring system that determines the reading level of books, article etc. and the reading comprehension level of students kindergarten-12th grade. http://en.wikipedia.org/wiki/Lexile
The scale goes from 0L to 2000L
Twilight by Stephanie Meyer is a 720L-lower than the range considered appropriate for 5th grade students (730L-850L)
Captain Underpants, at 850L, falls at the top of the range for fifth grade readers.
Among other books...
The Very Hungry Caterpillar-460L
Where the Wild Things Are-740L
The Catcher in the Rye-790L
Charlie and the Chocolate Factory-810L
Harry Potter and the Sorcerer's Stone-880L
Stardust by Neil Gaiman-970L
The Taming of the Shrew-1340L
I just found it amusing that an obsession for so many American teenage girls and their mothers is basically a fourth grade reading level.
EDIT: Wow, a couple of people were up in arms about The Catcher in the Rye being so low. Perhaps I can explain. The Lexile Framework describes the ability to comprehend the text. That takes into account vocabulary, syntax, complexity of metaphor, and all that good stuff. It doesn't take into consideration deeper meaning, a decidedly subjective undertaking. This is why something like Catcher, purposely written in the voice of not-so-bright teen that can't think of a synonym for phony, can score lower than Captain Underpants, written by a guy who packs his books with ridiculous superlatives because he's a little bit of a weirdo.