If you want to understand Sanders's strong showing, it's helpful to look at this chart. It shows that Sanders absolutely dominated young adult voters, in a way that even Barack Obama couldn't in 2008.
This NBC exit poll shows that 84 percent of Democratic voters between 17 and 29 ended up supporting Sanders. He also won the majority of the next age bracket up, those between 30 and 44.
These numbers are especially remarkable when you consider how they compare with Obama's 2008 Iowa primary. Back then, exit polls showed that he netted 57 percent of the same demographic. Clinton, by comparison, had 10 percent of young adults in the 2008 primary — a slightly smaller share than her 14 percent this time around.
The last Democratic primary, of course, was a more crowded field. Young adults had more candidates to choose from, and some threw their support behind John Edwards and Bill Richardson.
Young adults made up a smaller share of Democratic voters this time: They were 18 percent of caucus-goers in 2016, compared with 22 percent in 2008. Still, enough of them got behind Sanders that they were able to boost his performance significantly — and send him off to New Hampshire in a strong way.