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.