I don't think you're aware of the real implications of saying "a fandom is made out of teenage girls".
Firstly, we should ask ourselves: why do teenage girls/women always tend to be the punching bags of men in power and the mainstream media? And this is not a current joke btw, this predates the Beatlemania and it's not an exclusive issue that belongs to Kpop.
Secondly, I believe it is stereotyping women because it implies only teenage GIRLS would enjoy their music when the reality is completely the opposite. Hell, even on this forum we can find people from all ages/sex enjoying their music and other bgs music. Actually, implying a fandom consists of teenage girls also means we are not here for the music, but their looks, because they're handsome men and again undermining women's preferences. Come on, how many time haven't you heard people like bgs music bc of their looks? I know it can go the other way around as for ggs music, but it's not my focus here.
And even if the stereotype is on boybands, again why do we women have always to be the punching bags? We could say the joke ends up stereotyping both fandom and women as it implies women should know better. But I've heard countless times things as: "only teenage girls would listen to this type of music" "their fandom is made out of teenage girls", this kind of sentence has a double meaning bc it can be read as: teenage girls don't know any better about music and older women shouldn't be listening to this type of music.
Of course I know boybands have a target audience, but as a woman, I'm simply tired of having my preferences looked down on by people and the media and I find it completely offensive. If you don't, that's alright, but please DO NOT diminish other women's complaints because we know what these so-called jokes might actually be referring to.