Chait contemplates the Dems' advantage among young voters:
Democrats today must amass huge majorities of moderate voters in order to overcome conservatives’ numerical advantage over liberals. They must carefully wrap any proposal for activist government within the strictures of limited government, which is why Bill Clinton declared the era of big government to be over, and Obama has promised not to raise taxes for 99 percent of Americans. It’s entirely possible that, by the time today’s twentysomethings have reached middle age, these sorts of limits will cease to apply.
The Pew study reminds me of a great chart that the New York Times produced back in 2006 showing the effect that presidents have on brand loyalty to their party. 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.
(Chart from Pew)