Depends on your style. 21's not exactly old so you can probably get away with 'teenage' clothes, as long as it's not like a 14 year old or something. You said 'grungy' so that suggests young teenager.
But if you mean you're wanting to dress in a more 'grown-up' way, you could look at the way other people dress. Either people on the street or celebrities. Personally I always think Alexa Chung dresses nicely, she looks young but sophisticated I suppose. But that's just cos that's my style, you have to dress the way you want.