It is mainly about attitude though. The typical attitude we have seen from boomers about "kids these days" and "my generation was better" is not exclusive to boomers and also quite common in millennials.
Wiki explains:
""OK Boomer" is a catchphrase and internet meme that gained popularity among younger cohorts from 2019, used to dismiss or mock attitudes stereotypically attributed to the baby boomer generation. "
"The phrase "OK Boomer" is a pejorative retort used to dismiss or mock perceived narrow-minded, outdated, negatively-judgemental, or condescending attitudes of older people, particularly baby boomers. "
As you can see, it is not exclusively used on actual boomers.