The vast majority of children's publishers, including Penguin, Scholastic and HarperCollins amongst others, have backed plans to introduce age guidance for children's books from this autumn. A black and white design will appear on the back of books, near the barcode, with one of the categories 5+, 7+, 9+, 11+ or 13+/teen. These categories indicate reading level interest rather than ability - for more details, see the article Age-ranging gets the thumbs up from the current edition of the Bookseller.
As a PS to this, it appears that many authors are not at all keen on the idea. See Age guidance prompts author rebellion, also from the Bookseller (3/6/08) and Classification dismissed (Guardian, 6/6/08). However, Publishers press on with age guidance (Bookseller, 4/6/08). Watch this space!