There was no YA in my day (that's a good story, Grandma!). We were shunted directly from the children's section into adult fiction. Some of us got no farther than Science Fiction for some years, but most were left to wander the wide world of fiction in a Joycean peregrination. If we could get past the librarian (no small feat), we could take home anything that caught our semiliterate fancy.
Not so much, these days. The library where I volunteer has a Teen Room where the YA is kept. What happens to teens who make it to twenty? They are left to face all of fiction. Or were, but now we have NA...
Who are New Adults? Twentysomethings? Just those in college? Or is it just for vampires in college? Does my library have to build a new room for them? Very confusing. In any event, there would be no NA if some marketer somewhere didn't think he or she could reach potential readers and sell to them. That is the name of the game. Unless someone, somewhere believes a genre or book will sell to readers, that genre won't exist and that book won't be published. So we'll see where NA goes.
Bonus Answer: Know why there are very few books and movies set in the college years, compared to the years before and after? Because storytelling focuses on people in situations where they feel trapped. Readers and audiences want to watch people struggle to overcome their problems and strive for freedom. Think a high school student or employed person raising a family will feel sorry for a college student's problems? Yeah, nope.
Remember this reaction: "Wow, that person's really in trouble. I wonder how he or she will get out of it." Let that inform your writing. And your query letter.