Men - facial hair (not stippled, but real hair), well-toned or bulkier (no baby fat looks), no designer clothes (only high school kids wear predominantly label clothes. College students are notoriously poor, and can't afford that stuff)
Women - no baby fat, well-endowed (will sell the illusion better. sad, but true), mature haircuts, again stylish clothes, but not designer
And, of course, your actors' ability to sell the illusion. It is really hard to feign not caring about what people think of you and that comfort and self-esteem that you naturally get when you turn about 20-21. There are just some life experiences and natural instincts that can not be taught. In dialogue, syntax and cadence changes, gets better, and you become more comfortable with it. You are comfortable around people who smoke, drink, and do some things that maybe they shouldn't have gotten into. The truth is, it's more about attitude than anything that can be changed visually.