Taxpayers are rarely apathetic. Not sure if there is a demographic of the non voters....but I would wager they are in the most part, 18-25 and in school or at their first job....and the remainder are divided between those whose taxes are so low that they have no pressing interest in changing that or are not interested in politics period and the fringe who do not believe in our system of government.
Hold....wiki has something...
http://en.wikipedia.org/wiki/Voter_turnout