Perhaps the difference is that “okay, Boomer” went viral and became a motto, so it caught my attention in a way that the other didn’t. For example, “Millenials…”, after all, as far as I know, was never used on the floor of Parliament to dismiss an opposing politician.
Maybe the difference is that I never saw the “Millenials…” epithet used directly at a specific person (at least, not that I recall at the moment).
It’s possible that I see them differently because it’s one thing to decry a particular character trait that a person (or generation) has, and quite another to write off the person himself (or the generation itself).
Personally, I never used the Millenial epithet, and I do not agree with it, or with stereotyping in general.
Whataboutisms aside, I do think that using pejoratives reveals poor character. Same for not appreciating the value of an older generation.
And I reiterate that the Torah attaches great importance to respecting elders.