Each child is unique, and so is each book. Accurate judgments about age suitability are impossible, and approximate ones are worse than useless.
Children easily feel stigmatized, and many will put aside books they might love because of the fear of being called babyish. Other children will feel dismayed that books of their ‘correct’ age-group are too challenging, and will be put off reading even more firmly than before.
Age-banding seeks to help adults choose books for children, and we're all in favour of that; but it does so by giving them the wrong information. It’s also likely to encourage over-prescriptive or anxious adults to limit a child's reading in ways that are unnecessary and even damaging.
Everything about a book is already rich with clues about the sort of reader it hopes to find – jacket design, typography, cover copy, prose style, illustrations. These are genuine connections with potential readers, because they appeal to individual preference. An age-guidance figure is a false one, because it implies that all children of that age are the same.
Children are now taught to look closely at book covers for all the information they convey. The hope that they will not notice the age-guidance figure, or think it unimportant, is unfounded.
Writers take great care not to limit their readership unnecessarily. To tell a story as well and inclusively as possible, and then find someone at the door turning readers away, is contrary to everything we value about books, and reading, and literature itself.Reminds you of the nuances between our cultures when it comes to principles like the public good, individual liberties, and authority. I can't imagine the possibility of such a top-down decision being made by our publishers here, can you? At least, I hope not. You may declare your own dissension here -- it doesn't seem like UK citizenship is necessary.