The “young adult” sector is generally considered to encompass the 12-18 set. This is a pretty huge span: there’s much more of a developmental difference between, say, a 14-year-old and a 16-year-old than there is between a 30-year-old and a 32-year-old. Obviously, some YA books are aimed toward the younger end of the spectrum – the “tweens” – and some are pitched toward an older audience. Add to this that YA is beginning to encompass a few more years in either direction – so it might span from 10 to 25 (according to YALSA) – and that’s not exactly a homogenous demographic.
Additionally, there’s no switch that gets flipped when you turn 19, and all of a sudden you are totally uninterested in The Perks of Being a Wallflower and are picking up Jonathan Franzen’s Freedom instead. Readers in their late teens and early twenties may go back and forth between YA and adult fiction – especially if there isn’t much out there featuring main characters in that age range, as seems to be the case (“Where Are All the Young ‘Adults’?”, Young Adult Review Network; “The College Experience in YA Books,” YALSA’s The Hub). Both of those articles have a few suggestions for YA fiction featuring late teen/early 20s protagonists; I’ve added a few below as well. Feel free to add more in the comments!
Love is the Higher Law, David Levithan (set in New York; characters are high school seniors/college freshmen)
Prep, Curtis Sittenfeld (set at a Massachusetts boarding school)
I Am Charlotte Simmons, Tom Wolfe (set at a fictionalized Duke University)
The House of Sleep, Jonathan Coe (set at an English university)
Lucky Girls, Nell Freudenberger (short stories)