Truth told, I am not an expert in anything YA. At all. But I asked my trusty agent, and here's what she said: "I'm not totally sure, but my understanding is that there is not a hard and fast rule. But my experience if the protagonist/audience is 12 or younger, it is middle grade. Of course, the content can determine it too. If it's dark, it's YA."
She suggested asking a YA editor, but since I don't write YA, I didn't really have any contacts from which to pool. To answer your question, does it matter? I do feel like off the top of my head, most of the break-out youth set books have been firmly YA, but then again, I'm not an expert. The good news is that I run a blog with a lot of readers who know things when I don't. :) So if anyone can weigh in below, please do!