The idea that it was younger Democrats' liberalism is intuitive, since Iowa Democrats also split sharply along ideological lines. Sanders won "very liberal" Democrats by 19 percentage points, according to the entrance poll, while Clinton won moderates and conservatives by 20 points. Clinton topped Sanders more narrowly with "somewhat liberal" voters.
A deeper analysis of Iowa entrance poll results, though, shows that pattern held true for older voters, but not younger ones. Among those aged 40 and older, Clinton's margin against Sanders was largest with moderate and "somewhat liberal" Democrats, while Sanders performed best among the very liberal contingent.
The ideological dynamic among young Democrats shouldn't be over-interpreted -- sample sizes are in the low 100s, meaning sizable error margins -- but the lack of any greater support for Clinton among younger moderate Democrats suggests many are not supporting Sanders for his policies, but for his broader message and novelty on the national stage.
That interpretation is a bit of a leap, but not too far. Sanders has advertised not just a more liberal platform but also an idealistic call for "political revolution" -- something older voters may be more skeptical of after viewing many more political campaigns. A January Post-ABC national poll found Democrats under age 40 were more likely than their elders to trust Sanders on handling the economy and say he was closer to them on the issues, but they were also more apt to say he is more honest and trustworthy and likely to bring "needed change to Washington."
Regardless of the reasons, young voters' unified support for Sanders will be challenged by debates and upcoming contests. Moderate Democrats may be drawn more to Clinton as they gain more information about both candidates' policies, and African American and Hispanic Democrats who play a bigger role South Carolina and beyond have already shown greater support for Clinton.
Just the same, strong performances by Sanders may convince many older Democrats that he has a chance at winning the nomination and energizing the party's younger electorate, causing them to reconsider their steady support for Clinton.