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.

For sure the whole thing is much more fuzzy than this, with “in between” generations, like Jones gen and xennials being quite real. Plus all of the other factors that make up a society.

One big piece missing here though are the relative sizes of the generations, which is part of my interest. Boomer and Millennial bring the biggest, which colors the implications.

Yes, incredibly arbitrary, and yet that doesn't stop people immediately grabbing on to them for whatever tribalism they want to justify. The "baby boom" was a notable thing where suddenly there were a lot of post-war births, the rest is at best some vague suggestions that these people sort of behave similarly because of similar geopolitical conditions.