Mother Jones' Kevin Drum says younger voters have been voting Republican in greater numbers since the administration of George W. Bush. 60% of younger voters went for Obama in 2012, and 66% voted Democratic in 2008.
The problem for the GOP, says Drum, is a popular president inspires a sort of "brand loyalty" in voters in their 20's - leading them to mostly vote for that party the rest of their lives.
Basically, a popular president gains the votes of 20-year-olds, and those voters retain much of their loyalty to the president's party for the rest of their lives. The opposite happens with an unpopular president. So Democrats spent eight years with a president that 20-somethings liked (Clinton), then Republicans suffered through eight years with a president they hated (Bush), and now Democrats have eight years of a president that 20-somethings like again (Obama). That's 24 years worth of 20-year-olds who are likely to retain a fairly strong loyalty to the Democratic Party.
Obviously this could change. This is a tendency, not an iron law. But as these 20-somethings age, they're going to vote at higher rates and they're going to become more influential. And the likelihood is that most of them are going to stay Democrats. It's hard to overstate how big a headache this is for the GOP.