I think that's largely a modern phenomenon that we can thank technology for. Before modern communication technology, cultures were, by and large, very regional. But as technology began to allow wider communication, subcultures began to develop. You really saw it with pop culture, as people of similar ages would be listening to the same music, or watching the same TV shows or movies, things like that.

For example, without recording and broadcast technology, Beatlemania would have been impossible.

A lot of the "generational traits" that we see among different cohorts depend on shared culture. And modern communication is what makes that possible.