We all know what a Millennial is. There are stereotypes about what Millennials do and do not like, how lazy they may or may not be, and how often they check their Twitter feeds—all because we're comfortable using this single term to refer to an entire age demographic of the population. Millennial is a powerful word, and not because of the age group it refers to, but because of just how useful it is—just like Gen X or Baby Boomer.

There is no single or even typical way that generations historically get their names, because lumping everyone who's roughly the same age together is a relatively new phenomenon.

According to Peter Francese, a demographic and consumer markets expert, Baby Boomers were the first named generation to exist. (Those that came earlier, like The Greatest Generation that fought in World War II, were named retroactively.) It all started when the Census Bureau referred to the years between 1946 and 1964, during which birthrates rocketed up from around 3 million a year to over 4 million a year, as the "Post War Baby Boom." As the kids born in this boom started to grow into adults (and thus, consumers), ad agencies found traction by marketing their products to so-called Baby Boomers. This would be the first (and so far last) time a generation's "official" name would come from a government organization.

Eventually—as will inevitably happen to all of us, even the most maturity-challenged Millennials—the Baby Boomers got older and thus less appealing to companies with something to sell. The ad agencies wanted another catch-all term for the new members of their target age group and began shopping around different terms.

"They throw stuff at the wall and see what sticks," Francese says. "And in some of the meetings, they don’t stick." That's how Generation Y, a proto-term for Millennials, went in and out of fashion. "Generation Y was too difficult to say, too hard to brand, it didn’t have the cachet, it didn’t have the spark of Millennials," Francese says.

Not sticking is a matter of whether or not media organizations start using the term. And not just any media organization. "I’m talking about the Associated Press or Reuters—people who are syndicated that produce lots and lots of editorial content that they send out to various organizations," Francese says. As for determining the dates for Millennials, it all came down to demographics, and the old adage of comparing apples to apples.

"In 2010, which is when they did the census, Baby Boomers were all 45 to 64 years old," Francese explains. "Now, in order to compare Millennials to the Baby Boomers, because they're the next boom, you have to have what? Twenty years. And so in 2010, Millennials are people between 15 and 34. And then they work back from there to figure out when they were born."

If it seems like we're skipping over a generation, that's because we are. And for the most part, ad agencies did too. In 1991, Douglas Coupland wrote his book Generation X: Tales for an Accelerated Culture about the anonymity he and his contemporaries felt growing up in the shadow of the Baby Boomers. They were products of a 10- to 12-year downturn in birthrates sandwiched between the Boomers and the Millennials, and although the term stuck with the general population, the generation was the wrong size to matter much to marketers.

It seems unlikely ad agencies will take such a passive approach again.

"The ad agencies have a mission and an imperative to bring to their clients news of what’s going on in the marketplace," Francese says. " And so, inevitably, they segment the American populations into various groups. The necessity to do that means that they sit around and they come up with names."

The generation currently being born and growing up—the term Generation Z has often been used as a placeholder, though the Pew Research Center recently redefined them as Post-Millennials—is just beginning to acquire consumer value, and will become more powerful in the coming years. When that happens, ad agencies will have a perfectly workshopped label ready to slap on spending reports and style section columns.

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