Older voters still tend to vote in midterm elections. Younger voters don't, at least not to the same extent. And while I'm still sorting through the historical comparisons, there's no question that pattern held in this election. Here, via Matt Yglesias, is a graph showing the proportion of the electorate each group made up in this election and the last one:
As Yglesias properly note, this makes it more difficult to argue that the national mood has shifted in the last two years. If turnout had more closely matched what it was in 2008, Democrats would have done a lot better.