Well, like many have already said, there is really nothing wrong with hanging around with other people who are a few years younger than you. If you're afraid of that it would seem to make sense that you'd be afraid of the whole of Kumoricon as well.
It's not like everyone attending this thing is going to be kids, Nikkiolie is 20, and there are several others (heck, if I come I'll be a 21 year old, but I'm a guy, so that probably doesn't matter, lol). My point is, nobody is going to think anything about someone a couple years older hanging with a group that may have a few people who are younger. I think the only person putting that much pressure and paranoia on you is you (not directed at anyone specific, just whoever may feel like this event is inappropriate for them). If you really feel like it'd be a problem due to any secret fears or fears of intent, then just don't go. Go do something else. There are only like, thirty things going at any given time at this place; it's a great place to be, and you never HAVE to be anywhere if you're going to be uncomfortable with yourself.