Let's assume this represents a shift in the reading preferences of many adults. Some consider that current YA titles push the envelope in the area of sex and violence, and perhaps this is the lure of older audiences. I can only partly agree. "The Hunger Games" series certainly leaves a string of bodies in its wake, and though I haven't read any of the "Twilight" series, I would imagine hungry vamps would do substantial damage in that area. I don't believe the titillation factor of these stories is what draws adults. I contend it's their speculative nature. I haven't done any number crunching, but my sense is that most current YA novels have some element of the fantastical about them, and I would like to modify the survey to narrow down the titles purchased by adults.
So, as YA titles become bloodier and sexier and hotter-selling, a new category of fiction has sprung up. Dubbed "New Adult" this fiction slot is supposed to appeal to readers 18 to 30. Somehow, this sounds like adult fiction to me, because no one over 35 is targeted by the media anyway unless it's a Viagra commercial.
What elements push it from Adult to New Adult? The perspective of the young protagonist, apparently. In New Adult, our young hero must set aside the innocence of youth and step into the reality of adulthood. How un-fun? I suppose this is one reason I now write and read spec-fiction. The stories target individuals who want to transcend the realities of adulthood, or more broadly, humanhood, to explore the future and other worlds through stories that offer vision and insight into existence itself. Escapism? Most definitely, but also much more. And you certainly don't have to be any specific age to enjoy it.