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.
I just had this realization with a guy I'm training at work who lived in Queens and was 2 when 9/11 happened. He has absolutely no recollection of it. Meanwhile my old ass was in high school when it happened and I can remember that day very clearly.
Heh. "Highschool". I remember driving down the road and seeing cars abandoned in the middle of an intersection, doors open and all, just outside a popular diner on the corner. The crowd was eerily silent, packed around the TVs inside that normally showed the weather, sports, what have you. When I followed suit, a stranger turned to me and said "They hit the Twin Towers!" while behind him, a screen showed the second plane impacting.
That was a couple years postgraduate, and everything changed that day. Nothing seemed real anymore, and at the same time, everything mattered — down to a debilitating granular scale. I called a colleague who had just left for NYC on a photog trip, and he didn't pick up. A few days later, be called back, frantic and babbling about some crazy shot he got just walking around town on his own. It was too be TIME's cover.