TL;DR: it's just that the current state of the Fediverse is more attractive for that demographic than for most other people.
NTL;R: It's a damn complex subject but I'll try to simplify it.
Let's pretend for a moment that each user is a perfectly rational agent (they aren't, but it's a useful model). A perfectly rational agent will stay in the platform that offers him the most subjective value. And subjective value is tied to a bazillion of factors, among them:
- lots of content that the user wants to see, and it's easy to sort it out from things that he doesn't care about.
- lots of people whom he'd like to interact with, and it's easy to avoid people whom he'd rather would not.
- liking the interface and experience of the platform itself.
- the feeling that the platform is reliable, and won't suddenly stop working.
- agreement with the premises, goals, and values of the platform; etc.
Note that the weight of each of those factors changes from user to user, even among perfectly rational agents. For example, Alice might think "I'm fine with a shitty interface" (low weight for #3), while Bob might think "I can't stand an ugly platform" (high weight for #3).
Now, let's think about the differences between the Fediverse and "corporate media" in those points. For the first four factors, corporate media is clearly at an advantage, due to: network effect, network effect (again), age of the platform, and more money to throw at their user experience. For the fifth one, the Fediverse is at a big advantage, but only for users who care about open source and transparency.
And who cares about those things? Older, tech-savvier users, who are likely to also use Linux. For those, factor #5 weights so much that it compensates the cons of factors #1 to #4. But for the others, factor #5 is non-existent (they do benefit from the open nature of the Fediverse, but they don't weight it because they don't care about it).
That applies to the current state however. The Fediverse is growing, while Twitter and Reddit are enshittifying themselves; so over time there'll be less of a gap on the first four factors, promoting further migration to the Fediverse, even among people outside the demographic that OP narrowed down.
By the way, someone in Mastodon created a poll that confirms your "gut feeling" of most users being 30yo+.