There may be more than one Millennial generation: an older cohort that came of age during the George W. Bush years and identifies strongly with Obama’s progressive agenda, and a more skeptical younger cohort, whose political awareness has been shaped by Obama’s first term.However Obama won 75% of the votes of 21 year olds. These are kids old enough to remember the euphoria of Obama's 2008 campaign.
Younger Millennials went heavily for Mitt Romney in the 2012 election. The GOP nominee won 59 percent of 19-year-olds and 54 percent of 20-year-olds.
By 2016, four more one-year cohorts will have entered the electorate since 2012. Given the economically depressed world they've experienced, I'd guess many of these will vote GOP if they can hitch a ride with their parents to the polls.