Sarah, I do think if Biden wins the nomination that at the very least there will be a lot of stories about whether younger, more liberal Democrats turn out and vote at the levels Democrats need to beat Trump. And there are such big divides in support by age and ideology that it really doesn’t seem like a great thing for Democrats on paper.
But as Dan just pointed out, this works both ways — young people might turn out for Sanders but maybe more moderate voters don’t. Also, even though I think these types of stories are overblown — remember #BernieOrBust and PUMA (“Party Unity My Ass”) — modern elections tend to be so close because of partisanship, and even small effects along these lines can make a difference.