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.
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.
Is the main difference between a Young Adult and Adult book fiction is the topic of choice that is relevant to either Adults or young adults? So it won't matter.