Largely fitness level and attention to appearance (very-nice well-kept haircuts, probably a little male use of makeup, that kind of thing). Skin care routines (say, consistency of applying sunscreen and wearing hats when outdoors, from early ages on) can vary wildly with local cultural norms and, notably, with class-related socialization, with the result that some cohorts have much nicer skin in middle age than others.
Plus I've been in a few cities where, frankly, it just looks like the gene pool's exceptionally good.
[EDIT] Oh and stuff like serious hair-loss treatments and plugs and such, and skin blemish removal, short of full-on plastic surgery. I can just about guarantee those are more common in rich areas and richer cities than in poorer areas and poorer cities.
Los Angeles: 19 million people and home to Hollywood. It's to attractive people[1] what Silicon Valley is to nerds (not saying there isn't an overlap).
To be completely honest, I also incorrectly thought LA was America's plastic surgery capital - but it turns out to be Miami.
1. Who want to be in entertainment/acting, but being attractive is almost a prerequisite.
People find their tribes. A few years ago at work we were invaded by an intern army of what we affectionately called “dudebros”. They all had prince Harry style beards and talked about IPAs. By all accounts, the female interns were very polarized by the look.
Plus I've been in a few cities where, frankly, it just looks like the gene pool's exceptionally good.
[EDIT] Oh and stuff like serious hair-loss treatments and plugs and such, and skin blemish removal, short of full-on plastic surgery. I can just about guarantee those are more common in rich areas and richer cities than in poorer areas and poorer cities.