As a teacher, I'm faced with this question a lot in choosing books for my classroom library. I think, honestly, that there is a continuously growing "gray area" between the two genres. In my humble opinion, I think the reasons for that are two-fold:
1) At the middle-grade age range, you have a hugely diverse range of interests and maturity levels that change rapidly throughout the year. Students in my sixth grade class often start the year reading "Little House on the Prairie", by Christmas are reading "Freak the Mighty", and by the end of the year, they're devouring "The Hunger Games" Some of them start with "The Hunger Games" and end the year reading "Nick & Nora's Infinite Playlist" or even adult fiction. They're striving earlier to get to more mature content, based on the idea of "reading up" (5th graders don't really want to read about 5th graders, they want to read about 7th graders, 8th graders want to read about high schoolers, etc.) They're looking for stepping-stones as their tastes change.
2) Kids are reading more great fiction nowadays, and as they read more widely, their tastes tend to become more focused on genres of fiction rather than "appropriate age range". If a kid can't find a good first-person book about sports in MG, they'll look in YA - which can be a problem for parents and kids who might find YA is trending toward more mature themes, so there's an open door for MG fiction that skims the surface of a few of those more mature ideas, but not so much the graphic nature of the violence or explicit delving into contreversial areas like YA. The lines were blurred by the market demand, in my opinion.
I teach Honors Language Arts in the Sixth Grade, and you have kids who are looking forward to reading more challenging material, without the explicit (I use that word loosely to mean a deeper exploration of more mature themes) nature of many YA books. So, I will often recommend books that I feel fall into this UMG gray area to those kids, who will often be open to the new authors and themes within a genre they love, but without the "squirmy-because-it-feels-too-old-for-me" feeling.
I hope that made sense.
Thanks for the thoughtful replies. I definitely agree that market demand blurs the distinctions and categories and that many middle graders (by age) can and do read up to YA and adult. I remember my own reading at that age! UMG thus does seem to be a valid category.
I'm curious, though. Marketing aside, would you consider Hunger Games UMG or YA? It doesn't have sex, the themes are somewhat dark but not beyond anything most MG boys see in their video games and the writing itself is intelligent but not overly complex.
Also, and this goes back to my original post, how do you think agents perceive queries that describe the work as UMG?
For me, HG is in YA not because of the violence, but because of the romantic themes. If you ignore the violence, it's a story about a girl torn between two love interests. And then it's also a struggle against oppression, and other bigger-picture themes that I wouldn't expect in MG books.
As for upper MG - I would say you have MG books. Which are different from YA. And *within* the MG genre, you could describe it as being lower or upper. I wouldn't put age levels in (besides a general MG age range), because MG is all about varied reading levels within an age range.