"Generation X, commonly abbreviated to Gen X, is the generation born after the Western Post–World War II baby boom. Demographers, historians and commentators use beginning birth dates from the early 1960s to the early 1980s."
http://en.wikipedia.org/wiki/Generation_X
"Millennials, or the Millennial Generation,[1] also known as Generation Y, are the demographic cohort following Generation X. There are no precise dates when the generation starts and ends. Commentators use beginning birth years from the early 1980s to the early 2000s."
http://en.wikipedia.org/wiki/Millennials
"Generation Z is one name used for the cohort of people born after the Millennial Generation. There is no agreement on the exact dates of the generation with some sources starting it at the mid or late 1990s [1] or from the mid 2000s [2] to the present day. This is the generation that is currently being born."
http://en.wikipedia.org/wiki/Generation_Z
Alright so what have we learned?
Gen X: early 1960s to early 1980s
Gen Y: early 1980s to early 2000s
Gen Z: mid 1990s/early 2000s to today and further
You can't really have a clear start and end date to broad generalizations because the nature of a generalization prevents it.
When I hear "millennial generation", I think not of birth year, but rather of the set of people who grew up along with what we now refer to as the "technology industry".
We tend to take for granted nowadays that next year is going to be phenomenally different than this year, in terms of the tools that we use to communicate, have fun and get things done.
For "millennials", this is very easy to comprehend, because our whole lives are constructed around this constant change. I think it's much harder for people even slightly older (or later exposed to tech).
> defined by Pew as Americans ages 18 to 33