What is the Japanese term for an old man/grandpa who is into anime/is otaku? Is there a casual/slang term in Japanese for an old geezer who's a hardcore otaku? I know terms like erojiji and such but there are more referred to old men who are just perverted in general, but what about perverted old otaku?
Kitanai rōjin is what I have found about from your question.
It means dirty old man too if you want to translate.