At least according to Pew Research:
Pew Research Center has been studying the Millennial generationfor more than a decade. But as we enter 2018, it’s become clear to us that it’s time to determine a cutoff point between Millennials and the next generation. Turning 37 this year, the oldest Millennials are well into adulthood, and they first entered adulthood before today’s youngest adults were born.
In order to keep the Millennial generation analytically meaningful, and to begin looking at what might be unique about the next cohort, Pew Research Center will use 1996 as the last birth year for Millennials for our future work. Anyone born between 1981 and 1996 (ages 22-37 in 2018) will be considered a Millennial, and anyone born from 1997 onward will be part of a new generation. Since the oldest among this rising generation are just turning 21 this year, and most are still in their teens, we think it’s too early to give them a name – though The New York Times asked readers to take a stab – and we look forward to watching as conversations among researchers, the media and the public help a name for this generation take shape. In the meantime, we will simply call them “post-Millennials” until a common nomenclature takes hold.
Generational cutoff points aren’t an exact science. They should be viewed primarily as tools, allowing for the kinds of analyses detailed above. But their boundaries are not arbitrary. Generations are often considered by their span, but again there is no agreed upon formula for how long that span should be. At 16 years (1981 to 1996), our working definition of Millennials will be equivalent in age span to their preceding generation, Generation X (born between 1965 and 1980). By this definition, both are shorter than the span of the Baby Boomers (19 years) – the only generation officially designated by the U.S. Census Bureau, based on the famous surge in post-WWII births in 1946 and a significant decline in birthrates after 1964.
I've always been solidly an X-er, but some of my friends will be surprised to learn that they, too, are now officially Gen X.