I find Philip Pullman's discussion of age banding of books in the Guardian interesting.
Interesting because I work in an industry (video games) where age appropriate ratings are required on a product, and where creatively speaking, we are often forced by ratings boards to remove content due to "age inappropriateness". Don't pull the content, don't get the rating you want, ergo your sales may be affected because if you were a kids game, and you suddenly get a teen rating, moms won't buy your game for their kids. But the flip side is also true. If you shoot for a kids rating, teens may not buy your product because it seems too baby-ish for them, which could also hurt your sales. We could also have a very heated discussion about who is on these boards, and what gives them the credentials to determine what is "appropriate" for the rest of us.
The writer in me is actually relieved to hear that it's common not to write for a particular audience. Because when I go about writing a book, I have no idea who I'm writing for. My first book wound up being YA, probably for younger teens, but I didn't set out to write it for teens - the characters, plots, tone and length just worked out that way. And my second book is even murkier. I'm getting it critiqued on a YA forum, but it's a lot darker so who knows how it will eventually get marketed? But in a world where your niche appears to be more and more important, it can be angst-inducing to not know how to pitch a book until you've spent two years building it.
Novels and paintings now seem to be the last remaining art forms that aren't subject to ratings boards or self-ratings - movies, video games and even popular music now must exhibit ratings. I do not see why if it hasn't been broken for hundreds of years, we now need to "fix" the novel by adding age banding. It seems to me that such an idea has the potential to devolve into yet another system of ratings boards, and I personally don't feel like having some anonymous committee determine what I should be reading. I was a precocious reader. If I hadn't had access to "older" books, who knows if I might have lost interest in reading early on, due to the lack of challenge.
My opinion is that even for other media, a rating does not substitute for a parent or child's common sense. And based on the number of 12-year-olds out there I know are playing Grand Theft Auto (a Mature rated game), parents don't seem to care much about ratings either. I do have an opinion on whether GTA is appropriate for a 12-year-old, which, for the record, is "no". But I agree with Pullman. It's just my opinion. It's not a fact.