I think it also depends on where you're looking!! My husband and I just both turned 30 this summer, and my OSO is only 32 and has been doing polyamory/ENM on and off for a while with his other partner. There's another couple in town that my husband hangs out with to play D&D that is only a little older than us (both in their ~mid 30s).
I'm almost embarrassed to admit this, but after the pandemic started I really got into watching TikTok for feel good stuff since it was a depressing time. I'm not sure how much you know about TikTok but their algorithm is spot on and sometimes recognizes things about you before you do. I very quickly went from watching cute clips about animals and feel good news stories to clips with queer, kinky, neurodivergent, and polyamorous content.
I still get the cute clips (as well as some clips about rock tumbling and hiking), but the other clips have shown me that there is at least a small circle of polyamorous people that cater to that sort of content. Some of them are older, some of them younger (one as young as 22 and started in her teens!). Some of them are queer, some of them are not. There is even a variety in how they practice and what they find ethical or not! I honestly can't get enough now because there is an educational aspect to a lot of their videos as well and I am a lifetime learner.
As for when I was actively searching for another partner, OKCupid was my go to app and there were a variety of people on there as well. Admittedly, finding out that I was more than heteroflexible took place after I had stopped actively looking, so my experience from this only generated male candidates. Still, once again, past those who were not truly polyamorous or ENM, there was a variety of ages, though I kept my age scope no lower than 25 (I'd feel weird dating someone too young).
My final interaction (past here of course) is through munches that I attend through FetLife. A munch is a public, non-sexual gathering of folks interested in kink and/or BDSM that gives you a chance to get to know people on the scene and to hangout with your connections. It is somewhere else I am currently seeking community in, but more for kink, and I have not yet made many connections since it's relatively new to me.
Some of the munches are even specifically geared towards polyamory, but even the ones geared towards just kink tend to have quite a few polyamorous/ENM/open/swinger folks. Once again there is a variety amongst ages for sure, and I have even seen whole 'polycules' at such events.
That being said, above is also just based off of my own anecdotal interactions, so I'd be interested to see more concrete data about this before coming to any conclusions. Going just off my own observations I have noticed trends such as D&D/Table-top-gaming, neurodivergemcy, queer, and kink! Mind you these are things that interest/pertain to me so it's natural I would have found crossover there (a great case for correlation vs. causation). I am definitely not saying every polyamorous person is into these things, but from those that I have observed quite a few of them are from my own circles!
I'm sure there already is some research on at least some of this stuff, and I would love to see it, but I hope as it becomes more of a prevalent thing and gets more research it's way!
What a stimulating topic of conversation!!