The cutoff I've always heard, is if you remember 9/11. (Millenials are a primarily US phenomenon, so that would be a major cultural milestone.)
How old do you reckon someone would have to be to remember something like that? 5 years old? 10 years old?
Not to dox myself, but I was about five when the event happened, and my brother who is three does not recall it.
That would be damn near everyone, unless you mean people that don’t remember 9/11 are millennials, which is false. No one born in the 2000s could be a millenial, you must have been “growing up” into adolescence during the first decade of the new millennium, and must have witnessed the shift from the analog to the modern digital era.
Remembering the actual event would probably mean you were around 5 at the time, and a cutoff that gets thrown around would be 1995.
The people after 1995 are the next generation after millennials.