So...you're talking about the nineties as if it were an entirely new culture when it's...1990?
Incidentally, Y2K did feel entirely different from 1999. When the year 2000 started it was like a whole new era.
Talking North America here, I can't speak for other countries... from what I recall, the "nineties" didn't quite start until mullets stopped being an acceptable hairstyle, neon started to fade out, hair metal stopped being pop music, supplanted by grunge in terms of what kind of guitar music was popular... Also, when the only rappers were MC Hammer and Vanilla Ice--it was still practically the eighties.
You get a few "cusp" years at the beginning of a decade. I remember my older brother (born in '68) saying the eighties didn't quite start until '81 or so, when disco finally really died.
There has been an interesting stretch where the decade two decades prior became an object of fascination, and I chalk that up to it being a nostalgia thing for the generation that has "grown up" and started having children of their own, and missing being in high school. So in the 70s, you have fifties nostalgia with
American Graffiti and
Happy Days and
Grease; in the 80s you had sixties nostalgia with
The Big Chill and
The Wonder Years and Vietnam nostalgia like
Tour of Duty and
China Beach; in the 90s, the seventies came back with
Dazed and Confused and
That 70s Show and dudes growing sideburns again; and in the 2000s you had the rise of the "80s party". (I don't think there was a "forties nostalgia" period in the sixties; if there was, it has been forgotten.)
I see the occasional "Remember the 90s"-type post on Facebook nowadays. Grunge parties? Oversized t-shirts?
I'm looking forward to seeing what people will get nostalgic about for the 2000s. Dressing up like emos? Ugg boots, oversized sunglasses, yoga pants? Guys with their pants halfway down their butt?