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.