That's an interesting question and I wasn't sure, but I was curious enough to go look it up.

According to this source, which I can't verify the credibility of but which seems in line with what other sources say on the topic:

No official commission or group decides what each generation is called and when it starts and ends. Instead, different names and birth year cutoffs are proposed, and through a somewhat haphazard process a consensus slowly develops in the media and popular parlance.