Generations aren't about hard lines of division. For example, if some was born in December 1979 and another in January 1980, they would have more in common than with someone born in 1975 or 1985.

I was born six months before the millennial cutoff, but I find many of my touchstones align with millennials than with Gen X and then I have some that line up with Gen X.

Ultimately, the utility of generational analysis is degraded with pieces like this. There seems to be something useful about looking at how certain aged people relate to events, but trying to ask about "How millennials are ruining the work place for Gen X" isn't a good use of that analysis.

My understanding was that "millennial" was first coined to refer to those graduating in 2000.