I am not a statistician, so I cannot fully appreciate the extent to which the demographic studies go, but I have never thought those particular polls/data to be representative of what I understood to be happening when I decided to take note of what was going on...and that wasn't really until I was aware that I was going to be a part of that society pretty soon...that usually takes up to about 20 years for a person.
I'd say the generational thing needs to take more into account than statistical data from national census.
I agree but there is really no way to really perfectly nail down a time period of a generation. Even using a 20 year standard isn't that accurate because you are talking about people born in say 1965 and 1985 being in the same generation, Which they are obviously very different.
I think the best way to define the years of a generation is by which previous generation was the majority of parents during those years.
For example,
When the WWII generation was the majority of parents their kids would be the Baby Boomers,
When the Silient generation was the majority of parents their kids would be Generation Jones (What we know today as the late disco era boomers)
When the baby boomers were the majority of parents the kids are Gen-X
When Generation Jones were the majority of parents the kids are Gen-Y
Since Gen-X is so small we could probably merge the Xs&Ys kids into Gen-Z