Opposites or not, I think certain groups of people will be attracted to the same people, even if they are older. I agree with the statement that age doesn't matter.
"How do I be older fan?"
Older generically means 20+, but that's not the true meaning OR question of the topic. The question is about the term "fan". You can't tell ANYONE what an "older fan" is because everyone has their OWN opinion about being an anime fan.
I'm a fan of the anime style of drawing, but I've watched less than a dozen all the way through. I don't cosplay either, but I still study the practice of anime constantly. I've won multiple art contests throughout Kcon and Sak. That dedication is my version of being a fan. (
http://dshwshr55.deviantart.com by the way ~_^)
Acting out a cosplay might seem silly and even immature to an older crowd, leaving the little kids to do it, but there are 20+ fans that still love cosplaying, too. These older fans are still nevertheless fans of anime, and they ultimately show other older fans what fandom truely is.
Alternatively, many older fans of anime prefer talking about anime in their day and how kids today just eat up the next big thing. They're the real fans in their own eyes because they've seen a range of shows and have experience, rather than the tunnel vision of cartoon network.
The second point relates back to the thread title. "How do I be older fan?"
It doesn't specify a fan of anime. "Anime conventions" have become "anime/game conventions", and a fantasy/sci-fi genre is also merging now. I'm more a fan of video games than anime, and even art. These fans, like myself, enter the tournaments after years of practicing and they show what a dominant fan of gaming really is. Go have a talk with dancingtofu, enough said.
Still, the "older fans" of gaming are in the TTRPG room playing old school D&D and other dice rollers. Now THEY can spot younger distractions like cosplayers with a 6th sense (that's a very generic assumption, of course.)
So whether you cosplay and glomp or just like to buy manga and anime, whether you're a die hard tournament competitor or casual gamer, any of those can be considered a "fan". Can you ask someone how to be an "older fan"? Sure, but everyone will have a different view and answer for it.
Heh, it's long, but not super-long, so I'm safe. Hopefully this will be the last post on this subject.