Like I am pretty sure Astolat, founder of AO3 and also bestselling author Naomi Novik, is well past her thirties now, and she still writes fanfic on AO3.
Who do you think creates the stories you love? Definitely not all people under age 30. Why is it that they can become successful doing that, but for yourself it would be unacceptably creepy?
I’m sorry if this sounds combative but this is literally the sort of ageist rhetoric I run into with anti-shipper circles elsewhere on the internet. Those people use this as a cudgel for harassing people away from enjoying their hobbies, and you deserve better than that. If the idea of giving up fandom makes you miserable, don’t do it.
If you do think you’re outgrowing your current genre of fandom, then you could just consider trying out other genres. If you’d rather move out from Japanese animation, you could try J-Dramas instead. For me personally, (I’m not in my 30s yet but I will be approaching it within the next decade) I consume mostly nonfiction like historical books and documentaries, but once in a while I do pick up some new fiction that my friends recommend. Not all of them make me want to write fanfic, though, and that’s also fine.
But I don’t think you should quit fandom just because someone else on the internet thinks it’s creepy for people (often women) past a certain age to be in fandom. Adults are the people who built fandom.