Generations are divided like that, at least some of them.

Boomers for example, refers to the population boom after WW2 and millennials refer to people who generally came into adulthood around the widespread adoption of the internet.

Generally though, generations are divided into cohorts and they do have arbitrary cutoff points. Generally the goal isn't to say "this group of people experienced x event" it's just meant to say that the general cohort had similar over arching experiences in society.

That's also why there are mixed definitions, and over lapping time periods, because not everyone identifies perfectly with their cohort.