The classic plot in children’s literature pits an individual with a mission against an unrelenting individual or institution. And something of this kind has now occurred in the genre itself, except that, in place of Lyra Belacqua or Harry Potter, the passionate protagonists are Philip Pullman and JK Rowling and, substituting for Lord Voldemort or the Magisterium, we have Britain’s leading publishers of books for the young.

The cause of this war occurred in April, when the kid-lit wing of the Publishers Association announced plans to print a suggested reading age on all children’s books. This followed research apparently showing that many adults are wary of choosing junior volumes as gifts because of the risk of, say, giving a novel about an adolescent being hired as a drug mule to a sensitive eight-year-old.

But according to Pullman, literary development is variable. There are columnists who had been devouring War and Peace at six — while, routinely, there will be children in any classroom whose reading age will be a couple of years ahead of or behind the number of birthdays they’ve celebrated.

My biggest concern is that the move seems to have been motivated more by commerce than morality. All its statements emphasise that, by guiding nervous buyers through the bewildering shelves, this system will result in more books being sold. The other worry, as with all forms of intervention between the audience and art, is who gets to set the standards. Shoe size can be empirically measured but, even beyond variations in reading age, there’s the problem of competing sensitivities.

But, finally, a comparison with cinema is instructive in a particular way. It is now only with category 15 that the state begins to take an absolute stand on what people can see. The two lower categories — PG and 12A — leave it to the parents or guardians to make the decisions.

Those rules seem to acknowledge that late teenagers are more homogenous in their reactions than younger children. So, on this basis, the existing system of children’s bookselling — in which a general, invisible PG certificate applies to all titles — might sensibly be left in place.