Teenagers do wear sport shoes, sweatshirts and low-rise jeans, but usually with a trendy T-shirt and nice shirt on top. It looks like they grabbed the first thing handy, but they actually put a lot of thought into it -- boys and girls alike. Hard rock band T shirts are not "cool" in Paris.
Grown ups dress closer to what Americans call "business casual": Ann Taylor, Ralph Lauren, Banana Republic, and the Gap are some good examples of brands that will fit in in France. Classic styles are always recommended. Shorts and Nikes are fine to power walk through Paris in the afternoon, and grab a sandwich at a café, but you should change into khakis, shirt and leather shoes when going in a restaurant or bistro.
For men, a sport jacket with a dress shirt over a clean pair of jeans, and "dressy" leather shoes is fine for a casual night out.
For women, when going out at night, don't overdo it. French people prefer good cuts and pretty fabrics to beads and evening gowns. Nothing too flashy: think Armani, not Versace.
Accessorize: you don’t need to pack a huge suitcase. Different scarves, earrings and hairdos can change the look of a standard black outfit. But do pay attention to shoes, they often make a big difference to one's outfit. Sport shoes are a big no in Paris when going out at night...
More blog post on this topic:
French uses several verbs to say "to wear"
And to train on clothing vocabulary, check out chapter 4 of my free beginner level audio novel "À Moi Paris, une histoire au présent". The matching audio workbook features a long list of clothing related vocabulary.
