A little bit ago I showed why an older guy looking for a younger girl should search for them in a place where everyone is her age, and the guy is one of only a few outliers. It's much more difficult in a place where ages run the gamut. I chalked it up to how comfortable she would feel -- very much so when she fits in age-wise, uneasy when there's a broad mix, and not safe when she stands out as an outlier among fogies.
I still believe that description, but what's the source of her comfort or discomfort? I think it boils down to trust, in particular trusting that if she makes a move to "cooperate" (in Prisoner's Dilemma terms) by putting herself out there, it will be reciprocated rather than spurned. There's a great book, Coethnicity, that reviews the empirical results which show higher ethnic diversity associated with lower levels of trust and less provision of public goods. The authors propose many mechanisms that could drive this pattern, but they conclude that it mostly comes down to Zoe expecting co-ethnics to reciprocate her cooperative moves and to punish her if she shirks, while she doesn't expect the first or fear the latter reprisal from other-ethnics.
So in a highly mixed-age setting, if she belongs to the teenage ethnic group and you belong to the 20-something ethnic group, how does she know that if she makes a move on you, you won't view her as some weird immature little girl who presumes to be attractive to older guys? Plus she doesn't feel pressure from her own ethnic group or from other groups to approach members of other groups, whereas she feels she'll be ostracized by her group as a wet blanket if she doesn't approach people her own age (that is, if she "defects" from the game of mingling).
Thus, she's much more secure in approaching a boy of similar age because she expects greater reciprocity in the first place. Moreover, she feels compelled to approach boys her age in order to avoid the punishment from her age-mates if she were to not play along. And she's aware that the boys her age are going through the same thought process. So she trusts interacting with same-age boys more than older-age boys, even if she's equally attracted to both. When interacting with the former, there is greater norm enforcement, and trust is a precondition for having fun with someone.
That all changes, though, when most of the people are her age and you're an outlier. We expect guests to assimilate to our norms, at least as long as they're in our company. As the 20-something guest among the teenage ethnic group, you're expected to reciprocate advances, and you can expect them to quickly cast you out if you show disdain for their customs. Well now the situation is like it is when the boy is her own age -- the risk of being cruelly or callously rejected shrinks, even though the reward remains just as valuable, so she's more likely to indulge her desire to approach a cute older guy.
And of course, her worst nightmare is the opposite situation, where she's the teenage guest among people who are mostly in their 30s or older. This time, even if she tries to assimilate into the older group, she can't expect reciprocity from her target guy as much as before. The simple reason is that he feels incredible peer pressure to reject her advances, in part from male co-ethnics who are envious of him being approached by a pretty young thing, but more so from jealous female co-ethnics. More, she doesn't expect her own group to punish her if she doesn't make a good-faith effort to mingle -- they're not even there, and even if they were, wouldn't care much. And the other-ethnic group definitely won't punish her for not joining in -- if anything, most of them would punish her for trying to join. Her trust in the interactions now evaporates, and she doesn't approach anyone.
As I said in my first post, this applies to any setting, not just night clubs. Teenage girls will make moves on their tutor or teacher (assuming they find him attractive) because most of the people in that setting are her age. She's much less likely to in a setting that includes teenagers, graduate students, post-docs, and tenured faculty. And she's not likely at all to do so when most of the people are tenured faculty.
Again, it may sound strange to approach younger girls where you'll stick out as an outlier age-wise, but assuming you have any chance at all, that's where it's the greatest. * By taking the easy and more intuitive path of pursuing them where you'll blend into a broad range of ages, you allow trust-aborting diversity to spoil your fun.
* The same holds for any difference that can be construed as an ethnic difference, obviously including race: if you want a Cuban girl to feel comfortable approaching you, don't go to a club where Cubans are only one of a wide variety of races, but to a club that has a mostly Cuban crowd and you're the guest.