Establishing generations is a social invention, not an exact science. There's often disagreement among historians, marketers, demographers and social scientists about dates — much less names.

The Greatest Generation, those who grew up during the Great Depression, got named decades after the fact, based on the success of Tom Brokaw's 1998 book of the same name. Until then, they were generally known as the GI Generation.

"How names are created and stick is utterly random," said historian Neil Howe, who is credited with naming the millennials as the cohort that came of age around 2000.

Generation Z, now being used to describe the postmillennial generation, comes from an alphabetical sequence that started after the name Generation X stuck (also after a popular book, by Douglas Coupland, was published). The generation that followed was known as Generation Y for a time, until Howe's "millennials" took root.

In Howe's view, the lines between generations become more solid when considering what they don't remember. As he defines them, the Silent Generation (1925 to 1942) doesn't remember the Roaring '20s. Baby boomers (1943 to 1960) don't remember World War II. Gen Xers (1961 to 1981) don't remember when President John F. Kennedy was shot. Millennials (1982 to 2004) know only geopolitical terror and economic crisis.

As for Gen Z? "This generation will not remember anything before Obama and the economic crisis," Howe said. "2008 will be its generational dividing line."