Do you mean a fandom consisting of 75+ old fans, or a fandom that's 75+ year old?
The only such fandom (excluding literature) that comes to my mind is Star Trek. It's been around since 1966 I think
Then I suppose you could look into literary fandoms. Tolkien for example. Though I doubt you'll find any fans who are that old online. Someone in 50s, 60s more likely.
I liked tolkien and star wars in my early school days too. Then it was easier for me to create my own circle of hobbies. But I also have little idea how people of age get into fandoms. They probably don't advertise their hobbies.
I suppose those who know how to use the internet do it the same way we younger ones do. As for the other group - IRL fan clubs, or book clubs maybe? Whatever was a thing before the internet?
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The only such fandom (excluding literature) that comes to my mind is Star Trek. It's been around since 1966 I think