Technically if your parents are 'boomers (as in, born after WWII) you are a millennial.

The media prefers to argue about what years are what because then they don't have to think about it but the whole thing goes back to Strauss-Howe Generational Theory. It causes some weird contortions - my wife's parents are a Silent Generation and a Boomer so what's she, exactly? - but the reason for dividing it this way is that we're shaped by our childhood which is shaped by our parents and while the environment you grow up in might be keenly millennial if you're running it through the filter of your Silent Generation parents, you likely have a different experience than all your millennial friends.

Strauss-Howe is a sort of semi-historical Kabbalah in that it supposedly predicted every generation of Americans (if you take "Americans" back to 1640 and ignore the Civil War so really, it's about 8/10) but it's legitimately what caused Steve Bannon to back Trump and it's slavishly adored by a number of big talkers in finance so it's worth knowing the roots.