It's difficult to assess the actual turnout rates of various groups, in part because it is difficult to assess the both numerator (how many young people voted?) and the denominator (what is the size of the18-29 year-old Democratic--and independent?-electorate?).
We can, however, examine the share of Democratic voters who are 18-29 years old and then compare that number to the 18-29 year-old share from the previous election. An increase in this share indicates a greater increase in turnout among 18-29 year-olds, relative the change in turnout among other age groups. And, of course, the same method can be used for other demographic groups.
The data posted below come from an analysis of the 2004 and 2008 Democratic primary and caucus exit polls. Because I am using exit polls, the analysis is limited just to those states in which the 2004 and 2008 exit polls were administered.
Here's a look at the change in the youth vote:
Not so much, huh? Sure, there was an increase in some states, but not in most states.
What about female voters?
Pretty much an across-the-board increase in women as a share of the Democratic electorate.
Just "eyeballing" the data I don't see any clear relationship between changes in participation and candidate success.
Changes in the African-American share of the electorate:
Increases in most states, though not across the board.
Again, I don't see a clear pattern here between changes in the electorate and candidate support.
Finally, let's take a look at the overall patterns. The figure below shows the average change in groups as a % of the electorate, across states, from 2004 to 2008.
On balance,the greatest changes in participation have occurred among young voters and women, then among African-Americans, and there is virtually no average increase in independents as a share of the electorate.
An important caveat: these data include all (18) states for which comparable exit poll data are available, so we don't know what the differences are in the excluded states. Also, the data do not distinguish between open/closed/primary/caucus status, which no doubt plays some role, especially among independents. Finally, the data include Florida and New York, which are unique for obvious reasons.
Up next: Gaps, gaps, gaps!