As a rule of thumb, YA covers 13-18. Once you're talking 18+ or out of high school, you may be writing general fiction, or New Adult.
There can be exceptions to this... Jenny Han wrote a trilogy where the MC starts as 15 in the first book, 16 in the next, and is 18 and a college freshmen in the last. Or "Fangirl" is about a college freshman, and likely snuck in as YA for marketing purposes because its author was an established YA author so that's where her audience was. But in general, after 18/high school, you may be writing NA or adult fiction.