Most of Stockholm's nightlife is scattered to two different areas of the city. Birger Jarlsgatan, Stureplan, and the city end of Kungsträdgården are more upscale and trendy, and thus more expensive. Be sure to go to Stureplan, a few blocks up from the waterfront on any given weekend night, and you'll see crowds of sophisticated people gathering around Svampen (the Mushroom), the meeting place for people getting ready to go out in this upscale area. To the south, in Södermalm, things are a bit wilder and more bohemian, but no less hip. It's a more relaxed, casual scene where you'll find all types rubbing shoulders in the lively bars and clubs.
Many establishments will post and enforce a minimum-age requirement, which could be anywhere from 18 to 30, depending on the clientele they wish to serve, and they may frown on jeans and sneakers. Anywhere, a safe bet is wearing black, Stockholm's hue of choice. Things wind down around 3 am. Wherever you end up, a night of barhopping in Stockholm (and all of Sweden) includes fresher air now, because smoking has been banned since 2005 in all bars, clubs, and restaurants.