I went back to do a foundation degree before I got preggers and I was 20 - my friend called me a Dinosaur but it was just a joke, I fit in fine and I made some really good friends. Most of my friends are either years older or a few years younger then me. It doesn't matter about age, you either gel or you don't. I wouldn't worry about it, everyone will be in the same boat and hoping to make friends - unless you tattoo 'I'm 21, stay away' on your head..I don't think they're going to care much!