This is a somewhat surprising result. Per a Fox News poll, millenials strongly prefer Ted Cruz to Hillary Clinton by a 51-37 margin in a head to head matchup. Among these same voters, Clinton leads Trump by over 20 points.
As RedAlertPolitics notes, young voters were a massive driver for Barack Obama's general election win. If Hillary loses those to Cruz, it gives him a serious shot at winning in the general:
Cruz’s lead among young voters is fueling his general election chances in November. In this poll, Cruz defeats Clinton 47 percent to 44 percent in the general election; Trump loses 49 percent to 38 percent.
In 2008 and 2012, President Obama won younger voters by huge margins; he defeated Governor Mitt Romney 67 percent to 30 percent among voters 18-to-29 years old. If Cruz can neutralize or reverse this trend, it would be a huge feat for the Republican Party’s future.
Yet another reason for the Republican electorate to coalesce behind Cruz and reject Trump.