There's no official time frame, so you'll get different answers. Also, generations are somewhat arbitrary delineations that are defined by things other than age, particularly major events and themes that occurred in early life.

For example, it seems that "millennial" is congealing around 1981-1996, at least according to Wikipedia, but there's still variance. https://en.wikipedia.org/wiki/Millenial#Date_and_age_range_definitions

[–] Truck_kun@beehaw.org 2 points 10 months ago

Yeah, basically you're a millennial if you're currently in your 30's. If you're within a few years of that age, you may technically be a millennial, or even if not, you may still consider yourself a millennial, and that is fine. Generations are gradients, there is no firm start, or end, and as stated, is more about generalized experiences of your age group.

If you are 25 and want to call yourself a millennial, that's fine, if you want to call yourself a gen z/zoomer, that's fine too.