Kristen Soltis runs the numbers:
I've been troubled in recent months when discussing the issue of young voters with some fellow Republicans. There seems to be a sort of conventional wisdom that we should expect young voters to trend liberal and Democratic, that the behavior of young voters in 2008 is not serious cause for concern. This stems from a belief in partisanship as a life-cycle factor, that voters start liberal and Democratic and wind up older, conservative, and Republican. But the data paint a very different picture.
Take the graph of partisan identification for instance; over the last few decades, young voters have not identified with the Democratic party in substantially higher numbers than voters overall. Even conservatism had its moment among young voters in the 1980's. Yet with the end of the Reagan presidency, young voters shifted toward liberalism. This ideological shift did not play out into actual partisan identification in a meaningful way until 2006 and 2008.