Bachelorette Party for Under 21 year olds?
I'm not sure if you're from out of town or what, but there are plenty of venues of all types that are all ages. Some are fun restaurants with good attitude (and large bars), some are music venues with food, some are peep shows. The problem is most places don't really advertise their age rating, so it's hard to keep track, and once you're over 21 you forget.
As an example, the Great American is mostly all ages (they serve food), the DNA lounge has some 21+, some 18+, some all ages (the burritos in the back are, well, burritos). Slim's. etc.
Most of these places don't have much to do with food, or good food, though. Just about everywhere discussed on CH has a restaurant license, so if you're asking for a fun place to have dinner, you should specify the kind of food you like, price size of group, the usual.
The drop-dead obvious answer, though, is AsiaSF. They have a bachelorette page on their website. People say the food there is OK.