In one of the more pleasant surprises of this presidential campaign, young Americans are voting in big numbers, contributing to some unexpected results so far.

This is the first presidential campaign in which people age 18 to 29 make up the same proportion of the electorate as do baby boomers — about one-third. This year, the youth turnout for both parties in the primaries so far is rivaling 2008, the year of Barack Obama’s first campaign. On Saturday, young voters turned out in far greater numbers in the South Carolina Republican primary than they did in 2008 or 2012 in that state, according to a study by Tufts University’s Circle center. Donald Trump won the primary, with 32.5 percent of the vote, but young voters were the only group he didn’t carry. Marco Rubio came in second with 22.5 percent, closely followed by Ted Cruz, with 22.3 percent. Both Mr. Cruz and Mr. Trump drew bigger share of the youth vote than Mr. Rubio.

The youth vote’s biggest beneficiary by far is Bernie Sanders, who filled venues in Las Vegas with cheering young admirers last week, after winning more than 80 percent of this group in both Iowa and New Hampshire. On Saturday young people made up 18 percent of voters in Nevada’s Democratic caucus, five percentage points more than in 2008. Mr. Sanders again drew more than eight in 10 of these voters. Mrs. Clinton won Nevada with 52.7 percent, besting Mr. Sanders by 5.5 percentage points. But young people were largely responsible for closing what just a month ago had been a more than 20-point lead for her.

Many find it odd that the 74-year-old Sanders would have this appeal. But John Della Volpe wasn’t surprised. Surveying young voters in November, Mr. Della Volpe, the director of polling at the Harvard Institute of Politics, noted that support for Sanders among potential voters age 18 to 29 had rocketed from 1 percent to 41 percent in about six months. Asked what they valued most in a candidate, young voters said integrity, level-headedness, and authenticity, in that order. Political and business experience were far down the list.