And young people are warming to socialism - and, crucially, socialist policies - across the board.
No wonder it has been such a hard primary for candidates such as O'Rourke, who seemed to hop into the race banking on a boost from a boyish, punky flair that seemed to fizzle as fast as it flared.
How do we explain the affinity of young voters for old socialists - even with so many newer models on the market, and when so many strategists counsel against voting left or grey (much less both)?
The answer has to do, I think, with track records, radical critiques of American politics, generational alienation and a sense of political identity.
Older politicians have more opportunities to build track records, and those might be more important to the young than to voters of other ideologies and age brackets.
Why? Strong left-leaning track records - shared by Sanders as well as Gravel - offer two major benefits.
First, they bespeak a certain authenticity.
When Sanders showed it was possible to rake in young votes with staunchly leftist policies, plenty of centre-oriented Democrats began to show interest in things such as Medicare-for-all.
Roast young lefties for naivete if you must, but they seem to realise that a fight such as universal healthcare is going to require somebody truly invested in the idea, who's willing to take enormous flak over it and suffer a few defeats without giving up.
In short, endurance counts. Johnnies-come-lately inspire much less confidence on that front.
As 18-year-old Henry Williams, who is co-running Gravel's campaign, told me of his boss: "I think what we started to realise was this guy is where left activists are. He was where we are now a decade ago. He was trying in a time where American politics wasn't ready for him."
Aside from authentic commitment, candidates with long track records suggest they have developed a personal politics with a deeper historical scope - they don't think the problems in American life began when Trump was elected.
Nor do they believe that, before that moment, America was already great.
If your belief is that what's rotten in American politics stems from capitalism itself, then those sudden explanations of what went wrong don't make sense.
The explanations that ring true go back decades, and the people who have borne witness to them over time tend to be older. At the same time, millennials did not grow up in the clutches of the Cold War, and so today's Russia panic doesn't translate, for them, into a phobia of socialism, the way it might, perhaps, for their older counterparts.
It's also important to remember that the alienation between millennials and their parents' generation - baby boomers, largely - is genuine and laced with resentment.
Among the young there exists a real faith that boomers squandered opportunities to care for the environment, embraced austerity politics at the expense of needier generations and created a deregulated financial system that has left millennials saddled with debt and grim prospects.
Facing the world we're left with, why would today's up-and-comers look for solutions among the scads of boomer and boomeresque candidates cluttering the field?
Finally, youth is most attractive when you don't have it. For all the attention paid to Buttigieg and O'Rourke, younger candidates' pitches to their peers are destined to be met with a little well-earned scepticism.
If polls are any indication, young people have a concrete set of policy interests - action on climate, universal healthcare and free college for all - they hope to achieve with their 2020 votes, and the politicians making serious efforts to achieve them happen to be, likely for a variety of reasons, a little long in the tooth.
As more committed, young politicians such as Alexandria Ocasio-Cortez gain the halls of power, there will be more opportunities for the socialist-leaning young to vote for their own - though I suspect the very same characters who are unsatisfied with their voting habits now will be just as disappointed then.