In 2010 and 2012, turnout was highest among voters age 60 and older — both in Wisconsin and at the national level — according to data from the U.S. Census Bureau.

In 2010, when Walker was first elected, 29 percent of 18 to 29-year-olds voted in Wisconsin. Turnout was 54 percent among 40 to 49-year-olds, and 72 percent among 60 to 69-year-olds.

There is "overwhelming evidence" that an increase in age results in a higher probability of voting, Franklin said. Turnout continues to rise with age even into one's 70s and 80s, he said.

"The result of that is, while turnout among 18 to 29-year-olds went up noticeably beginning in 2004 and repeating that high turnout in ‘08 and ‘12, it is still substantially lower than the turnout of 40-year-olds or 60-year-olds," Franklin said. "The big question of turnout in this off-year election is not, ‘Will there be an age difference?’ There's always a big age difference. Really, it’s ‘Will the young people revert to their pattern in the past of substantially lower turnout in a midterm as compared to a presidential election?’"