Supposedly the cutoff was 64 = Boomer, 65 = GenX.
But I think on the margins it's always a bit tricky.
It's one of the reasons that they came up with the name "Xennial" for folks like me, born in '78. The big difference for our group is that we were analog childhood; digital adulthood. Meaning that we basically grew up without prevalent internet and cellphones for most of our childhood. But we were raised with computers and "came of age" during the internet boom, such that we were already primed to adjust to that societal change by the time we actually hit adulthood.
Contrast that with "pure" GenX, who largely didn't experience the internet until they were already adults, or "pure" Millennials, who were surrounded by cellphones and the internet essentially starting in their preteens and so never truly lived in a world without it.
I'd absolutely expect anyone between maybe 1962 and 1967 to be on the cutoff, and some are older but identify more with GenX, and some are younger but identify more with Boomers.