Charles Franklin has a series of graphs on age and turnout of which my favorite is this one, plotting the over or under representation of different age cohorts in the electorate relative to their share of the population. As you can see, the under-35 and the over-80 demographics are underrepresented in the population of actual voters. You can also see that there was a substantial change between 2000 (the blue line) and 2004 (the red line) in terms of turning out the youngest slice of voters. Because the Kerry campaign talked a lot about a youth surge driving them to victory and then they lost the election, there’s sometimes a tendency to dismiss the idea that their youth turnout efforts worked.
As you can see here, though, they worked just fine. Kerry both captured a large share of younger voters and got them to decrease the extent to which they’re underrepresented even in the context of a year when turnout rates increased across the board.