New Adult is supposedly (more on why that qualifier in a sec) a category that would cater to the 19 - 30 category (ish). So filling a gap between the experiences of the mostly high school characters of YA and the largely 30+ protags of "Adult" fiction. Sort of like the Chick Lit of the 90s did--these books would be about "finding one's way" or whatever. But with some other stuff thrown in with the stilettos.
That's not to say that there isn't or wouldn't be a lot of crossover between all of those categories, or that 19 - 30 year old people aren't "adults." It's just an acknowledgement that most people in those age groups have different concerns and life stuff than most 30+. That's what I think, at least.
Now, for that "supposedly," above. New Adult didn't exist as a term until about 5 years ago, when someone in the Biz literally made it up. Not that there's anything wrong with that. Every other category had to be made up at some point. But this is a new one, and it's had a hard time getting any traction.
Well, the biggest reason is that the categories used by agents and editors to describe something correspond directly to the categories in bookstores and libraries (yes, those are standardized!). This is because the hundreds of bookstores and libraries across the country (and digital retailers, too) order books based on codes called BISACs. They look like this:
|FIC009050||FICTION / Fantasy / Paranormal|
|FIC043000||FICTION / Coming of Age|
There are a lot of those. But there can't be too many, because if every subgenre and every potential age group delineation was coded, the codes would stop being useful. If everything got its own unique code...well, that's called an ISBN. And it's unique to each title. And there are millions of individual titles published each year. The whole point of grouping them is to make that number more manageable.
Adding a new category means that this entire system has to change--from how the bookstores arrange their shelves to how the publisher registers copyright. That, my friends, is not simple.
So the term "New Adult" hasn't really captured the hearts and minds of agents and editors because it doesn't exist in the Real World. Definitely the wholesalers and retailers don't use it. Readers don't know what it is, for the most part. Many view it as a cumbersome addition, fraught with the implication that 20-somethings "can't" read adult fiction for some reason (there's a reason Chick Lit is dead--it was boring and condescending after a while. Not all 20-something ladies are Carrie Bradshaw).
So, for now at least, calling a submission "New Adult" to an editor would be like calling it...Elephant In The Corner Fiction (it's, like, suspenseful, but for people who don't like surprises!). It means something, but it doesn't mean anything, really. It can't be sold in to retailers as what you're calling it. And that is a no-no!
All valid points, although I tend to think New Adult could be a real thing...BUT not without a huge shift in a huge system. It might seem like an update in a computer system, but it actually affects every link of the chain.