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Who are the millennials anyway?

Millennials: Who do they think they are? And more importantly, who do we think they are?

Millennials may be the hottest thing for demographers, brand specialists and marketers (this reporter can attest to hearing "is there a millennial angle?" asked in a newsroom). But it turns out there's surprisingly little consensus on who millennials actually are.

A review of various studies, surveys, newspaper articles and demographers' statements provides a wide range of ages that so-called millennials fit into. Based on these analyses, a millennial could be anyone born between 1976 and 2010, a 34-year time span that includes Apple's founding and the IMF's Greek bailout.

That means that depending on who you listen to, someone who was born when "Rocky" was in theaters is in the same demographic group as someone who was born when Sly Stallone was starring in "The Expendables," a movie that inherently poked fun at a bunch of aging, once-were action stars.

Even individual publications can't make up their minds—the New York Times has referred to at least four different ranges as the birth years of millennials.

Whoever they are, it turns out the younger-than-me-but-now-approaching-middle-age group might not be the tech-literate individualists we thought they were. A new study from Raketu shows that while three-quarters of millennials are concerned about the government's access to information on their mobile devices, 46 percent have "sexted" with others. Seventy-two percent have experienced "text regret," wishing they hadn't hit "send."

(Raketu defines millennials as ages 18 to 29, in case you were wondering. The company makes RakEM, an encrypted messaging app.)

Read MoreWhere millennials should go to school...and stay

Why is this important? Well, it's not really. Into which Sly Stallone era someone was born is of nominal importance.

But it does mean that every time you read the latest research on millennials, or advertisers spend hundreds of millions of dollars trying to capture the millennial market, there's a very good chance that they have only a rough idea who they're actually talking about or pitching to.

The term "millennials" could be referring to anywhere from fewer than 40 million Americans to more than 100 million, depending on the definition. Births in this time period peaked in 1990 near 4.2 million, and again in 2007 at 4.3 million, according to data from the Centers for Disease Control and Prevention.

Nonetheless, everyone we looked at agrees that if you were born in the years between 1986 and 1990, you're a millennial. Tag, you're it.