The list is definitely off.
I'm Generation X, and there's never been any question of that, but according to the list, only by two years. Which means
Thurston Moore isn't Generation X, nor
Mark Arm, nor, get this
Douglas Coupland who wrote the book that named the term to refer to his generation. Also leaves out
Richard Linkletter, who's movie Slacker is usually conisidered the defining film of my generation! We've had to either embrace or reject slackerdom ever since. These are the "Generation Spokesmodels" for X, so obviously the years have moved over time.
I think the media kept moving up "Generation X" for quite a while to encompass more people into slackerdom, since they obviously weren't boomers. Finally they dreamt up "Generation Y" to deal with the real kids when we slackers could no longer pass ourselves off as "youth culture", despite literally a couple decades getting away with it.
I remember hearing about it when Mark Arm turned 40 and realizing for the first time what it was like for the boomers when Mick Jagger did.
Before Coupland came up with Generation X (which I've always been annoyed with for somehow being lumped into
Billy Idol's first band!) we were widely proclaimed "The Pepsi Generation" on tv commercials all of the time.