Fandom Spaces are Adult Spaces.
I'm not sure why I have to say this sometimes, but fandom spaces are adult spaces. What we consider organized fandom was built by adults, for adults. But there are people who forget this. Like I've seen people admonish adults for being involved with fandom, saying "adults should be doing adult things" (whatever the hell those "adult things" are), and I've seen kids lament growing up saying they'll have to stop liking anime or comics or whatever property they're passionate about.
And I'm just like... no kid, that's not how it works. That's the opposite of how it works.
Modern organized fandom, as we know it, really starts with Star Trek. There were certainly fans of fantasy and science fiction before that — science fiction conventions have existed since the 1930s —but this was an inflection point where the culture we know today started.
Star Trek fandom galvanized in a way we hadn't seen before. People formed fan clubs, published zines, and organized conventions. And importantly, these were actions done by adults. To be more specific, it was dominated by adult women. Now obviously people of all genders were involved, but its undeniable that they were and remain the dominant group. Those zines were often filled with Spirk fanfic, and it's the origin of fanfic and shipping culture. Elyse Rosenstein, Joyce Yasner, Joan Winston, Linda Deneroff and Devra Langsam organized the first Star Trek convention in 1972, which was arguably the first modern media convention.
All of these things were done by adults, for adults.
And the successors of these fannish actions and organizations (the modern fandom sites like AO3 and the massively scaled fandom convention scene) are still made by adults primarily for other adults. It's not to say that younger people can't be welcome in these spaces, but they importantly are not centered.
Yet some people forget this.
There are a couple of reasons for this. One major one is that a lot of fan activity is online now. In the decades before widespread internet access, a lot of fan activity happened at in person fan club meetings. Adult nerds would meet regularly to socialize with other adult nerds. We saw the faces of our fellow fans, and knew more about their lives. When things moved online though, while we were able to connect farther and wider, these connections became more surface level. People were just avatars on screens, and there's a funny thing that happens when that's our reality.
We start assuming that everyone else is just like us.
We assume that people's backgrounds, opinions, and ages are similar to our own when it's not contradicted with direct informaition. A young person reading fanfic on AO3 starts to assume that the authors are their peers and not, say, a 40 year old office worker writing on their lunch break. The whole fandom world looks like it's also young people because they've never known better.
Secondly, our culture infantalizes anything done by women, and as I said repeatedly in this, women have always dominated organized fandom (regardless of what some CHUDs on social media pretend as they decry the latest comic book movie casting). That means that misogyny makes people think of fandom as childish, whether they realize it consciously or not.
The truth remains though: fandom spaces are adult spaces, and they only survive because they're adult spaces. The world of fandom does not end when you reach adulthood, it opens up.