From 2016 to 2021, nearly every state saw a spike in the number of young adults between the ages of 25 and 44 forming new households, a development with far-reaching implications for state populations, economic growth and infrastructure. For many millennials, the economic fallout of the Great Recession a decade and a half ago postponed what for many is a major milestone: leaving their parents’ house or housing shared with roommates to form their own household. But millennials began striking out on their own at a rapid clip starting about seven years ago, according to a new Stateline analysis o…