Pah this is so hard I'm trying to overhaul my wardrobe with about 300 pounds and I want to get all the right things because I usually just buy what's on sale so that I don't have to walk around naked but the clothes are never that nice :/
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.
get rid of jeans??? how could you! my mum is 50 something and still wears them! lol. My older friends wear skinnies and heels when they go out. my 24 yr old friend waore a cute uber short tartan skirt last time we were out. im 18 and i ware an all over silver sequin dress last time i was out! i havnt grown up yet! lol
Get rid of jeans at TWENTY?!!!!
I intend to wear them until I find a more convenient alternative (nothing else goes with my wardrobe as well as jeans do. god. so I'll be wearing them for decades yet...)
Try shirts? As in, not boring school shirts, but casual short-sleeved shirts and denim (is that the right word?) trousers.
Am only suggesting the above because I've been wearing a blue (casual... nothing I'd wear to work or school) shirt and white trousers and it looks professionally smart without screaming out "teenager!" I kinda understood why women spend so long in clothes stores when you do realise you can change your demeanour and how others perceive you simply by dressing well...