As for the rest (I'm about to show just how much of an adult I am, by the way)...
When I was growing up, there was no such thing as young adult. Yep, that long ago. I think the Sweet Valley High books were just coming into popularity (maybe), but by the time you're in high school I'm not sure those really cut it for reading material anymore. The point of this is kids jumped from reading kids' books to reading adult books. Age-appopriate had no meaning because the best a teenage reader could do was sift through the books and hope to find characters somewhere near their age. (Either that or so foreign that age didn't matter--I think this is one reason so many of us gravitated to sic-fi/fantasy early on.) Truly, the only book I can recall reading that would still fall under the YA heading today is The Outsiders.
I'm sure when someone first suggested YA there was some industry backlash. Where will we shelve them? Teens don't have money for books. All sorts of reasons to fight against it.
Then somewhere, some brave publisher decided to give it a try. And the books sold. Maybe not like hotcakes, but they sold. Over time this led to Harry Potter madness, Twilight insanity, and a few other huge sellers. YA wasn't some void--it was big money! (I can't find hard numbers, but I believe I've seen where YA as a genre is second only to romance in sales.)
There's a simple reason for this. YA is about firsts. First kiss, first love, first time, first car (er...firsts not in any particular order). A lot of things happen for the first time during those high school years. (Yes, some happen earlier or later, but on average, it's high school.) There were very few books that addressed those things. Things teenagers generally don't want to talk to their parents about too much. Teens needed YA fiction.
And the crazy part?
Adults needed to go back. Adult sales account for a very large portion of the YA market. Yes, part of it is well-written characters and blah blah blah, but the bigger part is wanting to "re-live" that time in our lives. For some of us (I'm probably in the minority) who loved high school, reading YA brings back a lot of good memories about the crazy shit we did. For others (who weren't so fond of the time), reading YA gives them a different (perhaps better) experience than what they lived through. It's very similar to the way a lot of women will return to romance novels after a bad break up. It's a reminder that there's good stuff out there somewhere.
Now New Adult is going through that same question phase that YA did. Do we need it? Who will it sell to? Is it just erotica for teens? Do we really want that?
The questions make me want to bash my head through a wall. Remember how I talked about the way teens used to jump from kids' books straight to adult? The advent of YA solved that...but there's still a hole. People (generally) don't go straight from high school graduation to a career or marriage and kids. There's college or first jobs or struggling as you move out of your parents' house. From eighteen to about twenty-five, there's a gaping chasm in fiction. Sure, you can sometimes find protagonists in the age bracket, but they aren't that common and there's no definitive place to look for them and even having characters of the right age doesn't guarantee a book that deals with those issues that plague "new adults."
Let me say it up front: New Adult is not erotica with teenagers. Period. Full stop. Books that are erotica with teenagers are erotica. Period. Full stop. It would be a very rare plot that is both erotica and deals with those new adult issues. (I can think of one that does, but it's a sub-plot in an adult book, so it doesn't really apply.)
So what is New Adult?
Young Adult is about firsts, but New Adult is about leaving. It's about exiting childhood, leaving that safety net as well as the one represented by parents. It's also about self-discovery. Those years after you leave home are when most people start to figure out who they really are without the rules of school or the rules of their family home. It's about responsibility on a level they may never have had to deal with before. In short, they're about becoming an adult in everything except age.
Does that include sex? Maybe. Probably. So does life though. And it is different than teen sex and different than married (or almost-married) sex. Self-discovery, remember? So yes, New Adult books will likely be sexier than their YA counterparts. That does not, however, make them erotica or even erotic romance. It just means sexier. Sort of like the girls in the 60s movies who go to school in their proper below-the-knee skirts just to roll them up once they get there and show a little leg. Sex in YA is (usually) fade-to-black or minimal details. Sex in New Adult is going to be more open--it's going to show a little more leg as it were.
Every time New Adult comes up on Twitter, I see people saying "we don't need it." Sure. We didn't need YA either. Generations of readers got by without it. That doesn't mean readers (and publishing) aren't better off for having it. It's taken self-published New Adult hits for publishers to sit up and take notice. People, readers, want new adult. They want to build a bridge over that gaping hole between YA and adult so they don't have to try to catapult across it.
And here's a crazy thought. Maybe, just maybe reading about the pitfalls of that time in life will help a few people from making the same mistakes the previous generation (or two) did. I know. Books don't ever teach people anything. Right?