Attached is a link for the 2004 election for 18-24 year olds. 41.9% reported voting.

http://www.census.gov/hhes/www/socdemo/voting/publications/p20/2004/tab01.xls

So, I think the posit that there was indeed an increase in the youth vote in 2008 is supported. Why? That's the million dollar question.