Entrance polls taken around Iowa as people entered caucus sites on Monday show that when it came to younger voters, Bernie Sanders crushed it: Those under age 30 made up the smallest group of voters at 18% of the turnout, down from 22% when Barack Obama overwhelmingly carried this group eight years ago. These young voters supported Mr. Sanders by an overwhelming margin: 84%, compared with 14% for Hillary Clinton. Mr. Sanders carried 30-to-44-year-olds voters by 21 points, 58% to 37%.

But 60% percent of Monday’s caucus-goers were 45 or older, and Mrs. Clinton carried them almost 2 to 1.

Many voters see Mrs. Clinton as the well-qualified candidate of the past and see 74-year-old Mr. Sanders, the oldest candidate running this year, as idealistic and focused on the future.