I started at Mac when I was 24 (I'm in second year now). I've rarely felt 'too old' and there are always people ten or twenty years older than I am in my classes as well.
I think a lot of it comes down to lifestyle. In addition to being in my mid twenties, I'm married and have a toddler. So I have zero interest in going out and partying. If you're into that kind of stuff you'll fit in just fine. If not, it takes a bit more work in joining clubs/activities to get to know people - but really that applies to EVERYONE in their first year of university, regardless of age.