This is Figure One in a document published by the White House, on Medium, called "15 Economic Facts About Millennials." It is included to establish a premise for the post: that the "Millennial generation will continue to be a sizable part of the population for many years." It seals off that generation at 2004, which means the next one begins at 2005. The next one is labeled without explanation: The Homeland Generation.
This data is credited to the Census Bureau, but presumably only the raw population numbers—the “Homeland Generation” is not, apparently, an official census designation. The choice to use it, then, fell to the people [...]