In one respect, at least, the Democratic nominating contest is running true to form: Sen. Bernie Sanders is getting the lion’s share of young adults’ votes. In New Hampshire, for example, voters under 30 represented 15% of the total but 28% of Mr. Sanders’s support. Voters 65 and older were 25% of the total but only 13% of the Sanders coalition.

The youngest voters haven’t always leaned left. In 1984 Ronald Reagan won 61% of voters under 25, more than his 59% of the popular vote. Something deeper, specific to our time, is...