It's a bit like the notions of pre-modern, modern, and post-modern. The change was continuous over a long period of time, but I think there are commonalities of thought and experience that tie generations together. Pearl Harbor, the Vietnam War, the fall of the Berlin Wall, 9/11; all of those influenced people of different ages at the time in different ways, compounding on each other.
The perspective of someone who was 18 on 9/11/2001 was different from someone who was 35, versus someone who was 50, versus someone who was 80. My kids, for example, have never known the WTC as anything other than an attack site, and recoiled when the NYC skyline was shown during New Year's Rockin' Eve 1999.
It’s that shift in perspective and experience that defines a generation and there are generation-bounding events like 9/11. But the period of time is not precise, and generally much longer than 14 years.
The boomers were defined by a demographic shift and the millennials (gen y, because Y comes after X and also because "y2k" ) were defined by being young-ish around New year's day 2000. Meanwhile X, z, α, and allegedly now "β" are just arbitrary postmarks who's locations are malleable and variable by the person you're talking to.
This is one of those cultural things that I tend to get grump and annoyed about because it's stupid and people pretend that it's real.
This is why people 'forget about gen x' imo, it's because there wasn't a single cultural event that aligned people in the gap between boomers (the baby boom, which resulted in a lot of people being a similar age to form a cohort) and millennials (wide spread access to rapidly developing internet, 9/11, and the dot com bubble happening during formative years)
That's just my opinion though, I know lots of stuff happened in that time, but I think those examples are standout events. (This is my perspective from the US, so things like the Berlin wall, I think had a less significant immediate effect on people here, culturally)