While Democrats had a modest edge with young voters for the last twenty years, starting in 2006 the scale of advantage has become significantly larger. From Gallup:

Trend: Party Affiliation Among Young Americans, Aged 18-29

Once individuals vote the same party a few times in a row the chance they stay loyal to that party goes up, so long-term this could become a real advantage for Democrats.

With young people leaning increasingly Democratic while seniors lean increasingly Republican, in the short-term this generational divide will give the GOP a big advantage in mid-term elections where turnout skews older.

This is what makes 2014 so scary for Democrats. Everything about the election from the design of the Senate, the shape of the House districts, and turnout patterns strongly disadvantage their younger, more ethically diverse coalition.

Democrats need to figure out how to get young people to vote in non-presidential elections as  well as getting serious about fixing our biased election system.