The most amazing stat coming out of the Iowa Democratic caucuses is this one: Among voters between the ages of 17 and 29, Bernie Sanders won 84 percent of the vote to Hillary Clinton's 14 percent.

That's astounding, even given the fact that we knew going into the caucuses that Sanders, a 74-year-old socialist, was clearly the choice of young people in the state. How astounding?  Barack Obama won the 17-29 vote by "only" 43 points in the 2008 Iowa Democratic caucuses. (Clinton finished third among young voters in Iowa in 2008; she got 11 percent of the vote.) Yes, Obama faced more — and more serious —opponents. But still.

If Iowa proved that Sanders is the candidate of Democratic youths, it also showed why young people aren't likely to carry him to the nomination. Why? There just aren't enough of them.

Voters ages 17-29 comprised 18 percent of the Iowa Democratic caucus electorate. (In 2008, they were 22 percent.)  That made them the smallest age group in Iowa, with 30-to-44-year-olds (19 percent), 45-to-64-year-olds (36 percent) and those 65 or older (28 percent) all representing a bigger chunk of voters.

Pay particular attention to those two older age brackets. Among 45-to-64-year-olds, Clinton beat Sanders by 23 points. Among voters 65 or older, Clinton won by 43 points. Her margins among middle-aged and older people effectively canceled out Sanders's massive win among young people.

Iowa is not terribly representative of the nation in many ways, but its median age — right around 38.2 — puts it right smack dab in the middle of the 50 states. That means that for Sanders to score clearer wins, he either needs to (a) win the youth vote by an even bigger margin or (b) limit Clinton's margin among older voters.

The latter option seems more feasible to me both because it's hard to win any demographic group by 70 points in a competitive race and because, well, Sanders is speaking to his own generation when it comes to voters over 65.

So yes, Sanders's numbers among young people are eye-popping. But they also show the limits to being only the youth candidate.