"Sexists say the same thing about sexist insults, and racists say the same thing about racist insults. This basically boils down to 'This insulting stereotype is true, so it's okay.'"

For someone with a name like Analyticturn, you're not doing your position any justice with a question-begging claim like this. Consider that this could be used to charge any statement perceived by anyone as insulting or stereotyping to any identity group with being discrimination or bigotry, regardless of the facts. Maybe instead of this you could make a non-fallacious argument for why and in what way the use of a phrase like "OK Boomer" is insulting or stereotyping.

Let's give your serious thought experiment the serious consideration it merits, though. A productive way to start this sort of conversation would be in asking where the harm is and to whom it's directed. Why is your example problematic? Does it really serve as a good analogy? I would suggest your example is problematic because of the history of marginalization that exists for black people, women, and the uneducated. These things are often tied together and intertwined in racist, sexist, and classist rationalizations. The harm is in perpetuating these stereotypes and denigrations of these identity groups.

Can you give me a parallel explanation for the harm being done to Baby Boomers through the use of a phrase like "OK Boomer"? I'm sincerely asking this because I can't think of one. It's not implying that Boomers are dumb, stupid, uneducated, or anything of the sort. That would be reading too much into things. At best, it's a response to dismissive or insulting behavior against young people, saying "You don't get it." Which is also one difference from the analogy you're giving. It conveniently ignores the context, as so many of these thought experiments and charges of ageism tend to do.

Go ahead and tell me how Baby Boomers are a protected class. Age is a protected class, but that's defined quite differently from a generation. In the US, only age 40 and up is considered a protected class, as if ageism is ironically limited by age. Your comments here are also willfully ignorant of the difference between teasing and harassment, as well as what is required by law for something to escalate to the latter. If you'd like to make an argument for why "OK Boomer" constitutes harassment, feel free. But you don't seem to care too much about making arguments that hold any water as much as you're interested in making assertions and questionable analogies.