I don't know who came up with that definition. Gen-Y is born late 80s and 90s. Millenials are born in the new MILLENIUM.
Gen Z, also known as iGen, was born in the new millennium.
The first time I heard the word millenial was 2017.
This world is weird. But in a world were people can choose their gender freely I think I can have the choice to use that word as I see fit. Feel free to do the same. ;)