I just read that the USA Millennials (ages 18-34) in 2015 surpassed the number of Baby Boomers (ages 51- 69) living by 500K (Pew Research estimates 75.4 million).
So what? Let me postulate by topical area:
Immigration: Amnesty, Acceleration, and Accomodation
Politics: Common ideologies in this group will frame the future social programs.
Consumption: Lots of houses, and furniture to be purchased (Harry Dent's book relives)
Culture: More diverse and less emphasis on minority differences.
Religion: More divisive views about role of religion (see: Millennials views)
Financial: More Debt, Inflation, and higher Interest Rates
Technology: Increased dependency on "intelligent assistants"
Family: More babies crying :) on planes
And the Bottom Line: Twenty years of echo in the making.