In the 2008 election, Barack Obama won over John McCain largely with the help of near record turn-out by a demographic that historically has not voted in large numbers: the 18-29 year olds. In 2008, 52 percent of the eligible 18-29 olds turned up to vote, and they voted overwhelmingly for Barack Obama.
In the 2010 midterm election, that demographic dropped off to just 24 percent giving the House of Representatives to the Republicans.
In the 2012 presidential election, again almost 51 percent of the 18-29 year olds voted to ensure that President Obama was re-elected. Without their support, he would have lost to Mitt Romney.
In 2014, however, only 19.9 percent of young voters showed up to vote giving control of the Senate to Republicans. It was the lowest rate of youth turnout recorded in the past forty years.
As a former precinct chair I have been painfully aware of the difficulties of getting this demographic to vote in midterm elections when a populist presidential candidate was not on the ballot. I personally tried really hard knocking on doors of young people and getting rebuffed.
I'll share two articles with largely contrasting viewpoints. The first (Huffington Post) provided a positive perspective of youth voters after the 2012 election when they did show up to vote. The second (Washington Post), provides a negative view of youth voters after the 2014 election when they didn't show up.
Huffington Post, March 20, 2013: The Social Generation: The Youth Vote and the Future of Progressivism
Washington Post, July 23, 2015: Where are all the young voters?
You can read the articles and pick out what you like and don't like. I have seen both sides of this demographic, but I have to say I was feeling especially bitter after the 2014 election, and cast much of the blame on this demographic. If I were to find one word to describe the 2014 youth "non-voter" it would be apathy.
Maybe both articles are correct. In 2012 there was still "hope". In 2014, that hope seemed more impossible as the "Party of No" continued to block Obama's initiatives on jobs and education. So many of them that I encountered blamed Obama and not Republican obstructionism.
Bernie Sanders, if nominated, has said that his political revolution would continue in 2018 if he hadn't achieved the necessary majorities, including a veto proof 60 in the Senate, in Congress in 2016. That may be commendable, but is it realistic? Will the Bernie Sanders revolution advance in 2018...or fall back?
In so far as Hillary Clinton is concerned, much of that demographic (maybe a third) will not support Hillary anyway, so maybe the question is moot in her case.