The idea of defining "generations" came from the baby boom following WW2. The baby boom is genrally defined as the 20 year period 1945-1964. From the the start of the rise in births following WW2 to the drop back below 4 million (IRRC) births annually. After that there have been attempts to define and name the following generations (and previous generations), more or less 20 years each. The genration following the baby boom was first called the baby bust and then later called Gen X. So it followed that the next one would be called Gen Y. Early writings sometimes called it the baby echo - from the peak years of the baby boom, 1958-1963, now having children).
I used to do a lot of work on generational preferences for churches. From what I can see, the only difference between one generation and the next worth seriouly considering as a whole was the one between the "builders" (born 1925-1944) and the boomers (45-64). Since then the generations have largely gone back to a more steady evolution.
Using the above model:
Builder: born 1925-1944
Boomer: born 1945-1964
Gen X: born 1965-1984
Gen Y: born 1985-2004
Gen ?: born 2005-2024
