Political Scientists have done studies that show political preferences are established early, and that the party a person votes for the first time they vote is very likely to be the party they vote for throughout their life.
You may wonder then -- why are a plurality of Baby Boomers Republicans? Wasn't that a generation that was very liberal -- opposing the Vietnam War, fighting for women's rights, joining the movement for minority civil rights, and making drug use more common (especially marijuana)?
The truth is that while many young Boomers were in that counter-culture movement, even more were frightened by it -- and those were the ones who established a plurality of Boomers voting Republican. And because of their large numbers, that Boomer GOP plurality has significantly contributed to Republican dominance in the past few decades. They still vote that way.
But there is a change happening. Generation X was more evenly split between Democrats and Republicans -- and the newest generation, the Millennials (those born after 1980), are significantly more Democratic than Republican (see the chart above from a USA Today / Rock The Vote Poll -- done between January 4th and 7th of a random national sample of 1,141 Millennials, with a 3.5 point margin of error).
It turns out that about 41% of Millennials (a plurality) identify as Democrats, while only 28% say they are Republicans -- a significant 13 point gap. And Millennials are not a small group. They are already the largest segment of the United States population (having surpassed the Baby Boomers in 2015). And as the Boomers die off, the Millennials will exercise increasing power over the electoral process.
When you combine this Millennial preference for the Democratic Party with the changing population demographics (whites becoming a smaller percentage of the population), the future looks very bright for Democrats.
Here is what the poll found about Millennial beliefs:
