it’s genuinely fine for people to be like 40 on tumblr the same way it’s fine for people to be like 40 on other social media sites. it’s just assumed that on twitter or facebook people will primarily interact with people their own age and in their social circles, or their interactions with people significantly younger than them will be apropriate (eg, ur weird aunt commenting “you’re a superstar! i know you can graduate college if you try!” on your selfie or something. dumb but harmless).
the thing that’s weird about tumblr is that people in their mid 30s and 14 year olds are both on here and in the same spaces, and those adults aren’t taking into consideration the power dynamic that comes with age or thinking about how appropriate their interactions with those children are.
modern fandom originated with middle aged women who were really into star trek, and that was fine because that circle was adults interacting with and making content for other adults. there isn’t anything fundamentally weird or creepy about grown women making fan media or really liking a piece of science fiction. the root issue is people having inappropriate dynamics with people significantly younger than them, which i really hate having to explain to begin with.