There is no "never" or "always" in the creative arts.
That said — and I am not
an expert in these matters, not even close — I think the problem with older characters in YA is that they're harder for teens to identify with, because their perspectives and concerns in life are different. College challenges are different from high-school challenges. Work-force challenges are even more different. Being in love starts to carry with it the serious possibility of marriage. Many older characters will be emancipated, living on their own, perhaps with someone else to help pay the rent, and parents aren't such an issue. Money, however, is
an issue, which can lead to jobs being an issue. Having sex isn't "should I or shouldn't I", but rather "Whom can/should I do it with?"
A few years ago, St. Martin's Press tried to create a new category for post-teen readers. They called it New Adult, and probably the canonical posting on the matter is this one: http://sjaejones.com/blog/2009/postadol ... t-fiction/
which includes this observation among many:
What makes YA compelling as a read is its immediacy; a young person cannot write of him/herself from any perspective aside from “now” and “later”. With a YA voice, the past is less present, the present looms like a storm, and the future ever just out of reach. With an adult voice, there is a sense that the future has come to pass, the past is present, and the present encompasses all that has been and all that will be.
There's also this more official definition from St. Martin's: http://sjaejones.com/blog/2009/new-adult-shelving/
I haven't seen any indication that New Adult caught on.
Nathan posted his own views on what makes a work YA in the site FAQ: http://blog.nathanbransford.com/2007/02 ... ke-ya.html