I would like to second this. I'm a programmer in my 20s, and my preferences run against the limos-and-clubbing stereotype. We do exist!
Chances are that there are some people who really like to go out on the town, some people who are indifferent, and some people who stay behind. And if this company employees great people, they will (1) treat you well no matter which group you're in, and (2) make it easy to tag along for the occasional thing that you actually want to attend. My coworkers are this way, and they are awesome.
As for the employees at the unnamed startup, it all depends on their personalities.