I don't think that the fediverse is exclusively used by "older tech nerds", but as someone who matches all three points you mentioned... I must say, you're still a good observer. XD

But it's logical. The more experienced tech crowd is the starting point of it. They are the ones not only able to see the flaws of corporate platforms and complain about it, but also with the technical skillset to just say "Fuck this, we make our own.". If you're not into computer stuff, you simply won't be able to create and maintain an alternative. And it also takes at least a little bit of both life- and coding / web / tech experience to get to that point, so the age is also a given, at least for the initiators. Younger folks may like what's happening and be joining in. And Linux runs the web. It dominates the server space, so the people who are working with it might also use it in their private life. Some others simply enjoy their OS and software not being bloated corporate spyware for the advertisement industry. So they are attracted early as well.

Don't worry though. "Older tech nerds" are regular people, too - with other hobbies and preferences, things, pets and people in their life. So the nature of the fediverse is... community. People stuff. And that is fully compatible with other demographics. If they have enough of the likes of Reddit and Meta, they will find a compatible alternative here for their needs. But that doesn't mean the fediverse has to replace those big tech platforms. People have choice, you know. And things can coexist. I'm perfectly fine with the size of Lemmy's community. Reddit refugees are highly welcome, but I don't worry about the user count, as long as there is a reasonable amount of interaction.