In essence, that is what this hot new genre is.
"New adult" is the old "young adult" but with sex in it.
It's what you once called "adult" but we can't have such a one-size-fits-all category in these modern times.
According to the savvy publishing reporters at the New York Times, it's all the fault of the publishing houses that we are afflicted with yet another genre. Those same publishers are operating from the theory that they'd hooked future adult readers with the whole Harry Potter series, which got them to read in the first place, and what better way to hold them than to create a new genre?
If the readers who began with Harry were to jump right into standard adult fare, they'd be liable to become lost in the tangles of complex sentences and deep thoughts. They could be disoriented by the situations presented, in which there are no vampires or werewolves or wizards.
And so, they would wander away, to become former readers.
Publishers are always open to anything that sells more books and increases their profits, so if readers want a different label for an existing category, they'll go with it. If that something has demonstrated success in the self-publishing arena, indicating a new trend, then it's a sure bet the big houses will take notice and take action.
The protagonists of new adult books are new adults themselves. Not unlike the majority of low-level workers in the publishing industry, fresh out of college and still filled with ennui. To say nothing of their dreams of financial wealth trickling away, causing much angst.
Clearly, such new adults should not be exposed to books in which the main characters are older than them. How could they relate to such as are not like them?
That isn't what fiction is for anymore.
We don't transition from children's books to teen books to adult books these days. We don't go looking for more complex writing styles as we mature, believing that we get smarter as we get older and so should our books.
Instead, we exist within our tribes, consuming tales about ourselves (ourselves being publishing interns in New York City struggling to make ends meet and pay off student loans and find love or at least a gratifying sexual relationship).
So there is "New Adult" for the new adults in the world, who don't want to risk reading about some middle-aged woman facing a crisis such as they have never experienced.
Wouldn't want to stretch the mind or expand a horizon. Who's buying that these days?