I don't think that's why they chose those dates.

Boomer were born at the end of the War.

Gen X were born after the enactment of Civil Rights Act of 1965

Millenials were born after the end of the recession and the enactment of voting rights act and the expiration of the period for states to ratify the equal rights amendment.

Each event, or combinations thereof, are viewed as major changes in he trajectory of American politics.