It is absolutely
not widely agreed upon, and frankly, the 80s born aren't millennials acceptance/understanding is only growing. If anything, Millennials start in the late 80s, or even as late as 1990, and run through around the end of the century give or take a few years.
But don't take my word for it:
https://www.good.is/articles/generation-xennials
https://socialmediaweek.org/blog/2015/04/oregon-trail-generation/
https://medium.com/@thehipp/fuck-you-i-m-not-a-millennial-e92e653ceb39
https://www.nytimes.com/2015/02/05/opinion/wait-what-im-a-millennial.html
And I could go on and on posting links to people opining or pushing back on the 80s generations somehow being Millennials. Labeling generations is stupid in the first place (and god help me if I see another "how to interact with millennials" article written by some out of touch 50-something generic business person) but at least try to get people grouped together with some sort of similar cultural and adolescent experience and identity.
The gist of all those articles is "people keep saying that those born in the early 80s are Millennials, but
I don't feel that way." Each article is framed as pushing back against the typical convention. Thus, the very reason those articles exist in the first place is that
the Millennial generation is typically understood to to start with people born in the early-80s.
I could equally post link after link supporting this, but this isn't the forum for that. Like the true Millennial I am (born in the late-80s, btw) I'll just cite
Wikipedia. And don't you go editing it...
And yes, everyone is different, but this doesn't change the fact that the thing we call the "Millennial Generation" is typically defined as those born in the early-80s (who "came of age" at the turn of the Millennium) to those born in the late-90s to 2000s.
As far as Millennials starting "in the late 80s, or even as late as 1990", Mark Zuckerberg was born in 1984. He's pretty much
the definitive Millennial.
Fine, then come up with a different term for people born 1990-2002ish. Because they're different and distinct and shouldn't be grouped with the 78-89ish set.
Just as it's widely agreed-upon that those born in the early-80s are Millennials, it's equally widely-agreed upon that those born in the late-90s to early 2000s are the start of the post-Millennial "Generation Z". It's a stupid term, yes, but that's just a placeholder until we come up with something better.
My definition:
If Facebook existed when you were in college and you can remember 9/11, then you're a Millennial. That defines the "media, technology, and world experiences" of the generation.