Honestly the way we think about generational cohorts is a little bit of novelty. I think generational cohorts really started being focused on with the boomers. My opinion is that these categories are used by marketing campaigns. They because a "thing" when the marketing industry identified that they could target Boomers to acquire lifetime customers/voters/users/followers. So in my opinion, and I may get a lot of flak for this, is that Millennials are Gen Boomer 2.0. I know it has its limitations but I think if your parents are Boomers which stretches from 1946 to 1964 you can count yourself a millennial with some reasonable exceptions due to really old parents who might have children over their 60's.

I pulled this out of the air, it's just my opinion based from biased anecdotal evidence from my own experience and my friend's experiences.