Based on the way I've heard young teens use the phrase "Ok, Boomer", it seems to be a putdown aimed at people who are too young to be Boomers but are expressing Boomer-type opinions/attitudes.
Dictionary.com says: ”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 closed-minded opinions associated with the Baby Boomer generation and older people more generally.”
IOW it's the currently hip way of calling someone an old fogey, Especially if they're not really old.