How do you define generations within a culture?

[Mnementh]

Nymeria said:

https://medium.com/@Sarah.Jane/are-you-a-millennial-a-hipster-or-just-a-hypercritical-bully-does-it-even-matter-59a3776f2b7f

Got me thinking about the mabiguous nature of "generations" as outlined in the US.  Have the boomers, X, and millenials being th ebig three that dominate current culture.  In particular Millenials get a lot of press online and in print.  In simple terms these generations are defined by set years of birth, although depending on who you ask these can have a bit of wiggle room.  Due to this I find Millenial is often used in disparging terms towards "youths" with the classic characteristics of selishness, laziness, and entitlement.  In fact many people who would qualify for this group, mainly the older ones within it, reject that they can be a part of it because of the connotation.

• According to Iconoclast, a consumer research firm, the first Millennials were born in 1978.
• Newsweek magazine reported that the Millennial generation was born between 1977 and 1994.
• In separate articles, the New York Times pegged the Millennials at 1976-1990 and 1978-1998.
• A Time magazine article placed the Millennials at 1980-2000.

What do you think? Is a generation simply people born between year X and year Y, or does it come down to shared cultural norms such as the dawn ot television for boomers and the proliferation of the internet for millenials?

Obviously it is a lot easier and clearer to define it by years. On the other hand, it makes more sense to look into cultural borders, as this influences stuff. Luckily, for a given country this is basically the same. You can define a year after Internet became widespread, or after 9-11. So I would say the best is defining by years, but choose the significant years by cultural effects.