I just have to say, you do know that any adult under 40 is a millenial don't you?
This may come as a bit of a shocker to you, but you have no idea what you're talking about.
Pew Research Center, which has a bit more authority than most, submitted an article on why they think a birth range of 1981~1996 (or in 2018, ages 22~37) are "millenials". And it's basically based on being 5~20 years old during the 9/11 attacks, which a lot of other groups base their deals on as well, rather than say for example growing up during the aftermath of the 1982 recession or remembering life during the Cold War or the AIDS epidemic. And I'm sure that isn't any sort of biaist choice in events by 25 year old annalists
at all.
Generation X, popularized by MTV and Pepsi, covers the mid-1960s to early 1980s and Harvard defines it as 1965~1984. While Generation Z covers the mid-1990s to mid-2000. Generation Y, aka the "millenials", should follow suit and cover a 20 year age group of 1985~2005, which really would have put 9/11 as a highlight of their life. But we don't actually have that. Instead 8 year olds on social media to college seniors are simply lumped into the "millenial" category along with middle aged adults. Which for example, is why you think every "adult" (16? 18? 21?) under the age of 40 counts even through we're talking about three generations here.
I base my scale on something a little different; The social mentality. Like Gen X was known to be a bunch of cynical latchkey children with no goal or aim in life but in midlife they have a happy work/life balance while being some of the hardest workers in the US since WWII. While Gen Y is mostly known for growing up with interpersonal communications, freebie rewards, the sudden increase in narcissism leading to SJW crusades and refusing to take the blame for anything. And their only current life accomplishment is the legalization of the psychogenic drugs they use to make them selves feel better.