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:
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:
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.
Here's where I've most noticed folks in the 40s and 50s attending: ACX meetups; Winter Solstice
This sounds to me like: "don't visit random rationalist meetups, focus on the exceptional ones", because the busy people will probably follow the same strategy.
(I assume that by "ACX meetups" you meant the yearly "ACX Everywhere" meetups.)
And the other part could probably be achieved by asking every rationalist over 40 what other social places they are visiting, and checking those.
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.
I'm not available, I'll be clear about that up front, but I am in my late 40s, in case that helps anyone update their priors about the demographics. YMMV as to whether I'm intellectually interesting ;-)
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.
As well as the generic suggestions people are making in the answers, it seems like you might be able to get more specific suggestions if the question specified whether you're looking for long distance vs. nearby/in-person dating, and (if the latter) a rough idea of where you are located.
The rationalist community is a nice place to meet interesting, intellectual men, which makes it potentially a good dating pool for straight/bisexual women. However, this community is young: most people seem to be in their 20s or 30s. For middle-aged women like me, that's a problem. What are some other communities where it's easy to meet straight/bisexual men who are: (i) intellectually interesting is vaguely similar ways to how the rationalist community is intellectually interesting (ii) middle-aged and (iii) available?