If we're speaking only of what I think should be part of the collection of themes and devices that make up the YA genre, then I think college is a huge factor. It's impossible not to notice what a pervasive issue college admissions is for teenagers and how it seems to come into focus earlier all the time (I'm thirty and I really didn't give serious thought to what college I would attend before my junior year. Now, you hear stories about kids signing up with college admissions consultants in ninth grade). It affects decision-making on all manner of extra-curricular activities and its freighted with all sorts of financial consequences. In short, college is as big a deal for teens as prom, virginity, drugs, and your best friend (to name a few other members of the thematic pantheon). So, to me, it makes sense that not only the anticipation of college but the realization of college (at least freshman year) should have a place in fiction for young adults.
But, the reality of the situation is, in my experience, that bookstore buyers--the people who make the stocking decisions for independent stores and chains--are uneasy with YA extending beyond high school. It's not a hard and fast rule. For instance, Laurie Stolarz's Red is for Remembrance is set in college, and it's sold many tens of thousands of copies in the YA section. And, of course, John Green's An Abundance of Katherines is set in that extremely interesting summer between high school and college. Other examples are out there, but the key thing to recognize is that there's always something to mitigate or overwhelm the college concern. In the two examples I mentioned, the books were follow ups to successful titles whose stories were based in high school. We've also had success with books about post-high-school experience other than college (Barbara Shoup's wonderful Everything You Want is a case in point). Other examples of mitigating factors might include a really strong subgenre angle that has widely acknowledged YA appeal (if you had a really commercial dark fantasy werewolf novel, it probably wouldn't make much difference whether the heroine were 18 or 19, a senior in high school or a college freshman). But if you're trying to sell me a debut coming-of-age story about a girl from Minnesota in her first year at Swarthmore, that's going to be a very tricky proposition for me.
A couple caveats. This is my experience with Flux, only. I haven't made a study of this with other editors. I'm also sympathetic to the buyers' positions. Realistically, they have to draw a line somewhere. Shelf space is not infinite.
In short, it's best to give this issue a lot of thought.
UPDATE Brian Mandabach's comment is too good to go unnoticed:
"Just because it's not likely to find a spot on the shelf in the teen zone doesn't mean it won't find a shelf somewhere else. One idea is to not think of your book as YA or anything else--just think of it as your novel, the story that you want to tell. Make it as great as you can, then think about selling it. I would say don't listen to death knells from the market--listen to the story in your head and heart!!"