Here is an interesting post that takes a stab at sorting out the various age levels your novel might fall into.
I have no intention of offering a definitive answer to the question. I think anyone who attempts to do so is at best deluded, and at worst being deliberately deceptive or oversimplifying the case. It simply is a fuzzy issue. What I can do is offer you a very practical way of discerning juvenile from middle grade and middle grade from young adult fiction.
That seems like a promising beginning! Indeed, it *is* hard to sort out, though in fact your agent or editor may indeed help you with that. And we see that the author of this post — Austin Hackney — asserts that there is no such thing as “a book a ten-year-old would like to read,” which is true, as that sort of thing depends entirely on the ten-year-old in question.
Here, then, is the practical distinction offered in this post:
Children’s books: The purpose of the story in terms of the protagonist is to reassure the reader that “all is well with the world . . . Adventures may be had, risks may be run, threats and dangers may be faced – but in the end, all is well.
MG: The purpose of the story for the protagonist … is to find where she belongs, to locate herself, to understand her place in the family, in school, in the village, on the street, in the world. It is the quest for belonging. . . . Many protagonists in middle grade fiction begin their stories feeling anxious and lonely, and end their stories feeling at home, feeling loved, surrounded by a supportive network of friends and family.
YA: The purpose of a young adult story in terms of the protagonist is that she should discover her individuality; that she should come into her own personal power, self-realization, and autonomy within the world. Furthermore, having become fully individuated, she should then employ her power to cause dramatic and lasting changes to occur in the world around her; to make her mark.
How about that? Kristen Nelson of Pub Rants defined the difference between MG and YA thus: in MG, the protagonist may have adventures, but at the end, he or she is still a child. In YA, the protagonist takes the first irrevocable steps into adulthood.
How does that definition work with the above? Could be an aspect of the same kind of thing.
Here’s another, don’t remember where I saw this one: In MG, the protagonist save the world. In YA, the protagonist figures out tough relationships. Such a definition lends itself to YA-as-basically-romance, a trend which I hate; and it seems at odds with the MG-is-about-forming-relationships idea in the post I linked to.
I don’t read enough children’s literature to have an opinion about that. Or MG, really, though it seems to me it’s true that an awful lot of MG stories start out with the protagonist separate and lonely, and end with the protagonist in a much happier place regarding family.
For YA, I think it’s basically true that the story is about irrevocably leaving childhood behind; which could be expressed in terms of self-realization and autonomy. But I certainly doubt that the YA protagonist is required to effect lasting broad-scale change on the world. Surely a lot of stories are going to be smaller scale than that, especially contemporary stories such as basically all the contemporary YA I’ve read — Saving Francesca, say, and Five Flavors of Dumb, and The Improbably Theory of Ana and Zak.
Quick: what are a handful of YA fantasy / SF novels in which the protagonist does not effect broad change in the world? I can think of one: The City in the Lake is much more about maintaining or restoring the proper order than effecting change. A lot of high fantasy is about restoring the proper order of the world, isn’t it? What are some other YA fantasy / SF titles that do or do not fit the change-the-world idea?