Smoking a lot, not advised! Or drink a lot, advised! :P
If you're going somewhere where you need to look a bit older, change bow you dress a bit? buy a leather jacket/ blazer?
Don't worry about it. It gets annoying but it can actually be useful. I'm 24 and can still get a child's ticket on the bus! However it is a bit degrading when a hairdresser asks you if you've broken up from college now (this happened last week!).
Wearing makeup and more mature clothes might help but in reality there's not much you can do. You'll be glad when you're older!