“OK boomer” is “a viral internet slang phrase used, often in a humorous or ironic manner, to call out or dismiss out-of-touch or close-minded opinions associated with the Baby Boomer generation and older people more generally.”
As an "older" millennial/Xennial I have observed. The Boomers call anybody younger than them "millennials" and the Gen Z's call anybody older than them "Boomers". Gen Z gets called Millennials all the time when they do anything dumb (aka Tide Pod Challenge, among many others). It's almost hilarious how many things my generation has been blamed for.