In anime, people sometimes use the words "aunt" or "auntie" to refer to a young female character. This is in fact a bit insulting, because in Japanese culture, calling a female a word that is considered to be old(er) is rude.
My question is, is there a similar culture in English-speaking country? What's the proper English words to use if one is going to convey such an insult? Can a native speaker understand it if I use "aunt"?
The corresponding word in English is grandma or granny. You can also use grandpa or gramps for a man as an insult. While these words are obviously fine to use for your grandparents, using them for other adults is almost always insulting (even older adults).
In more recent slang, you can also call someone a boomer (like in the expression ok, boomer). Note that this is insulting even when referring to someone who is a Baby Boomer.
Not "aunt". That word is used literally (to mean the the sister of a parent) or in an extended way (a close female friend of a parent, who may babysit etc). Unlike (say) Japanese, English tends not to use family words like "grandmother", "aunt" or "sister" to mean "old woman", "middle-aged woman", "young woman".
There are ageist insults in English. Like much slang, these may be dialect, or local language. "Old biddy" (faltering old woman) or "Old fogey" (old, unfashionable man). Macmillian has a list I've heard expressions like "Young fogey" (a young person who dresses or has social attitudes like an old man), but these are not common - Don't use them.
An interesting modern term is "a karen" (from the name, Karen) which is an insult for a white interfering, middle-aged woman with conservative attitudes. This is very modern slang, and it will probably be out of date soon, as teenagers move on to the next bit of slang.
Finally, I've heard "you're such a mum", used to mean "you don't do the cool things that other teenagers do". But again, if you're not "one of the cool teenagers" you shouldn't use that expression.
