People do think about age differently in America and you hit it spot on with people moving out early, becoming more independent, etc.
But we dont think of age 25-30-35 as old or anything. It's not that we think people who are 30 shouldn't hang around 21 year olds, it's that 95% of the time, the 30 year olds don't want to hang around them. Clubs with 18 year olds and the 21 year olds are generally very much different as far as vibes and everything go. An older girl or older guy simply can't stand the younger ones because they're so wasted all the time, being super belligerent, very aggressive, etc. I'm only 24 years old and I never go to clubs that allow people under 21 in. There's this college bar that I like to go to a lot and even there I feel super old around 21 year olds. I look at the girls and think, no way is she 21. That's my sign that I have no place being there on a Thursday (student night). In Keco's case, I imagine she gets approached on the dance floor every 5 minutes by a drunken (older or younger) guy. It's hard to enjoy yourself as a girl in that environment if you wish to dance solo. So that's why I'd assume she wouldn't want to go to those types of clubs.
As far as clubs go, a girl dancing on the dance floor is prey. If you go up to her and start dancing with her and she doesn't push you away, that's telling a guy he's "in", regardless if the girl thinks otherwise. It's a jungle out there lol.