According to the U.S. Census in 2002 (where n=26,712) 45% of 18 to 24 year olds were registered voters and 32% actually voted.
This and other statistics of voting and registration can be found at http://www.census.gov/population/www...o/voting.html.
The data above came from this table
, which offers several breakdowns of the data.
I was never good with stats (been putting that crap off too long now
) so I'll leave it to your sister to glean the data she needs... it appears that everything she'll need is there though.