There is a movement in the UK to "band" children's books by age: 7+, 9+ and so on. A website has been established by authors, librarians, teachers and parents in opposition to this trend.
Our local Barnes and Noble organizes the children's section by age, as most bookstores do. Amazon allows you to search by age as well. This seems to be much the same thing, although I agree with the opponents to banding on the idea that having a big "7+" sticker on the front of a book deters older children who may still enjoy the story, far more than how a bookstore shelves its books.
Honestly I don't think parents need this kind of additional information to help their kids choose books. Sure, it would be easy to say "okay honey, you go buy anything you want as long as it's banded under 10+", but that's just lazy parenting. I've never bought a book (or a toy or a movie) for my boys asked for without examining it first, and I've never needed a big age sticker on the front of a book to be able to tell if it was appropriate or not. If I'm on the fence and we're in the store, I make a promise to come back and get the book or buy it online after I go home and do additional research - I've never had a freak-out over that, although I have compromised - on a wet, rainy Saturday, perhaps - and agreed to buy another Hardy Boys now, and research the new book later on.
Age banding isn't the way to go because it could discourage children from reading "young" books. More importantly it doesn't work because it substitutes another person's judgment for that of a parent. Discouraging parents to do their homework and pay attention to what their children are interested in is an unhealthy recipe indeed.