First, some commentary.

The rationalist community is fairly young. Although there are a few folks in their 50s and 60s, most folks are under 40, as you've noticed. I've only recently joined the over 40 crowd, and there are some effects that make us less present than younger folks:

• we have more established lives that pull us away from in person rationalists events
• we are more likely to already have fulfilling relationships (romantic and friendly) that reduce the pressure to attend parties to meet more people
• we have less energy for staying up late at house parties

This means we show up less often to things compared to younger folks. That said, we're not totally absent. I manage to show up to things from time to time. Here's where I've most noticed folks in the 40s and 50s attending:

• ACX meetups
• Winter Solstice

Aside from those, as others have noted, rationalists have overlaps with other communities, and many rationalists, as they get older, seem to spend more time with those communities rather than attending rationalists events. Stuff like that includes things in dance, kink, poly, queer, and authentic relating events.

Dancers and analytic types have a surprising overlap. What the demographics look like is heavily dependent on the local scene, and really this is only good advice if you like dancing.

Should you be so inclined, in my experience with polyamorous and kink communities, you get a lot of overlap with the nerdy, generally intellectual, neuro-atypical, etc. communities. I’ve observed a decent age-diversity, especially in the kink communities. I’m 46 year-old white man, and I haven’t really felt out-of-place—I know, big surprise. 😬

Poly communities have tended to lean to liberal politics, while swinger communities that I have encountered lean more conservative.

It’s not too uncommon for people to be weird in multiple ways.