I would quibble with the notion that increased youth turnout necessarily helps Sanders.
This is because I also quibble with the (widely-expressed) notion that Sanders supporters attend his rallies and post support for him on social media and then don’t bother to vote. (I’m sure there are such people but not enough to be a major factor.) I think the people who attend the rallies and post on social media do vote, but these people are a small -though very visible - minority of the population even in that age group, and the vast majority don’t do these things. What’s driving low youth-voter turnout is that the non-rally-attending young people vote at much lower levels than non-rally attending people in older age groups.
IOW, the cohort of young people is more dichotomous than older people in this regard. There are more passionate fervent political junkies and also more apolitical people.
But here’s the thing, as applied to Sanders. Politically passionate young people tend to be drawn to revolutionary ideas, causes, and people. This has been the case throughout history AFAICT. And this is the basis of Sanders’ strong support among this cohort. He’s the guy who is going to remake the system and lead us to a Brave New World utopia. But it doesn’t follow that Sanders has nearly the same level of support among the more politically apathetic young people.
In sum, people are making the assumption that if younger eligible voters voted at the same levels as older voters, that Sanders would receive the same level of support as he does from others in that cohort. But I don’t that assumption is necessarily correct, and it’s likely that among those marginal additional voters his advantage would be significantly smaller if it would exist at all.