As Per wikipedia:
New Adult: This category is intended to be marketed to post-adolescents and young-adults ages 18 to 30. This age group is considered to be the lucrative "cross-over" category of young-adult titles that appeal to both the young-adult market and to an adult audience. Publishers of young-adult fiction now favor this category as it encompasses a far broader audience. The chief features that distinguish the new-adult fiction category from young-adult fiction are the perspective of the young protagonist and the scope of the protagonist's life experience. Perspective is gained as childhood innocence fades and life experience is gained, which brings insight. It is this insight which is lacking in traditional young-adult fiction. The other main differences are characters' ages and the settings. YA does not usually include characters over age 18 or in college, but these characters are featured in New Adult books. New Adult can best be described as the age category after Young Adult.
Young Adult: is fiction written, published, or marketed to adolescents and young adults. The Young Adult Library Services Association (YALSA) of the American Library Association (ALA) defines a young adult as someone between the ages of twelve and eighteen. Authors and readers of young adult (YA) novels often define the genre as literature as traditionally written for ages ranging from sixteen years up to the age of twenty-five, while Teen Fiction is written for the ages of ten to fifteen. The terms young-adult novel, juvenile novel, young-adult book, etc. refer to the works in the YA category.
The subject matter and story lines of YA literature are typically consistent with the age and experience of the main character, but YA literature spans the spectrum of fiction genres. YA stories that focus on the specific challenges of youth are sometimes referred to as problem novels or coming-of-age novels.
***I can tell you one thing. Having done several book signings, the readers (or in some cases, the MOTHERS of the readers) define these two categories quite well while perusing my books.
Just about every time, they will ask: Does this have strong language or sex? What about blood and guts gory? Those seem to be the questions that define the separation of the two categories. LOL
