Growing up doesn't really end at age 20, so why should young adult fiction stop there? The New Adult genre, a growing subset of young adult lit, aims to give voice to the post-high school experience and its implied transition to independent living: college, moving away from home, traveling, starting first jobs and even sex. The content may be darker and more mature than what is traditionally found in YA, and the protagonists range from late teens to early 20s, but the stories offer many of the same kind of identity challenges and coming of age narratives as their YA brethren.
Once upon a time, there wasn't much market potential for YA short stories. These days, though, digital publishing has made one-off narratives an appealing format for readers and a powerful marketing tool for publishers.