Cool is relative so its not more about behavior and than fashion, but dressing age appropriate does help. Seeing a woman in an outfit that seems to young or too old seems uncool. A woman who exudes confidence and treats people with respect is always cool do matter what she
wears or no matter how old she is.
You don't need anything fashionable. The best and easiest way is to be confident and to be happy with yourself. It's not the clothes that make the person. If you emit a confident, happy person then you will seem like the person that others would want to be around, and I guess that's close to being 'cool'
Wear comfortable clothes and underware -otherwise it shows on your face. Never be in a hurry- leave plenty of time for everything. Have a sense of humor. You have to have enough confidence in yourself to never be overwhelmed by anything. Don't bite off more than you can chew. Be happy with yourself and what you do regardless of what other people think- once you try to impress someone, you've automatically failed.
the only way to look cool is to be happy with yourself: if somebody told you to wear something to be cool but then you don't feel confortable and confident wearing those... well.. everybody will noticed and you won't look cool at all..