I've thought about this before: why is there so much focus on high school? I have a few possible answers that might apply, or might be part of a larger answer:
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High school kids have easier conflicts. When we're less mature and less sure of ourselves, we make bad decisions. We invent social conflicts, we look for rash solutions to simple problems, we have unrealistic wants and desires. All of that makes for easy plot potential. There are also great plots for adults, but they take a little more to set up, which isn't ideal for a collaborative story. Take even the most basic conflict: Sarah McMillan vs. Karyn Black. Would that level of pettiness make any sense for two twenty-somethings working in the same office? You'd have to come up with an excuse as to why Sarah is so immature. In high school, we just accept that she is.
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High school is the age when most people wake up sexually and start exploring these sorts of genres. I have no idea what the actual proportions are, but I'm sure there's a decent representation on this website, both in readers and in writers, from the teenage demographic. I know from my own personal experience, the very first TG story I wrote was when I was 19, and I was deliberately trying to avoid writing about high school, so... I wrote about a college freshman. I just didn't have a lot of other experience to draw from yet.
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High school is an easy, accessible setting. Almost all of us have gone through it, and it's pretty similar in any country. You can throw any kind of character into that setting, because everyone goes to high school. But if, for example, we start talking about a Jon who has grown up to become a real estate agent, suddenly people who don't know about real estate have less to contribute. Or if Jon is an artist, then non-artists are lost. High school student is familiar enough and vague enough that everybody can add something.
Which is both good and bad, I suppose. Like Hikaru said, you're not the first person to come up with an idea to write the characters older, but if you have an idea you like, you should run with it. Hope other people join in, but don't expect them to. You need to set up a solid foundation before others will be comfortable jumping onto it, and I hope you don't take that as discouragement: I'm sure you're very capable of creating a very strong one. Just go for it. If you need to focus on just one branch for a while in order to figure out your ground rules, that's fine, but once you've done that you might be surprised how well other people pick up what you've developed.
In any case, I hope you keep writing, and I hope you find success in it :)
