The generations are determined by what the biggest common experience they all have is (at least, that's how it's supposed to be.)
Millennials are millennials because they all remember the turn of the millennium. anyone born 1997 or later wouldn't remember it, which is why the generation line was drawn where it was.
There are people who find that weird and prefer to call anybody born after 2000 gen Z because they were born after the turn of the millennium, so there's a sizable amount of people who've taken to calling anybody born 1996-2000 a "zillennial" as a compromise. I use the term sometimes, but only when I need to demonstrate to somebody that there's no clear difference between a young millennial and an older gen Z.