New adult (NA) fiction, also rendered as new-adult fiction, is a developing genre of fiction with protagonists in the 18–30 age bracket. St. Martin's Press first.
Young adult fiction (YA) is a category of fiction written for readers from 12 to 18 years of age. While the genre is targeted to teenagers, approximately half of YA.
We've talked plenty about young adult fiction here on Writer's Edit. We've covered how to master conflict and what mistakes to avoid when writing for young.