I'm not trying to generalize young people with this....
But I think Bernie appeals to the young, under informed voters because of his lack of specificity. When I was young, I automatically assumed most of what Bernie is calling for was 100% the right thing to do. His policy proposals are so simple and they don't require a lot of thought. Healthcare is too expensive. We all agree. So, let's make it free! Everyone else has made it free, why shouldn't we? College is too exepnsive, let's make it free! So, for this segment, I think his lack of actual details helps him. Because, the moment he would actually have to go into details about how these things are going to happen, they're going to tune out. They just don't care. He's offering simple, perfect solutions, so fuck ya! Get rekt! Werk.
So, if you're not super informed on the actual act of governance, and you have one guy promising the moon and one woman saying "Well...that's not going to happen...." it's no wonder they go with the first option. It's the optimism of youth, I think. (And, I'm not saying all Bernie supporters are young or anything like that, I'm talking specifically about why he's more popular with the chillren.
Also, it's popular to like Bernie. Herd mentality is a hell of a drug.