Age is a problem, yes; Bernie's other problem, though, is that he is seen as too unorthodox (moderate, rather than the extreme, to reduce things to a linear scale) on key issues for much of the Democratic base. He's too progressive on economic issues for the neoliberal elites, but that's mostly a problem in getting resources too build a national following if you don't already have that, and while it was a drag in the runup to the 2016 primaries when Bernie wasn't a national household name and the most popular politician in the country, but it's not much of an issue for 2020.
> Sure 45% of Democrats really really really love Bernie
As of April (the most recent I can find) Sanders had 57% favorability nationally (not just among Democrats.)
> I cannot imagine the people who want to dismantle the ACA would swing over and vote for "free education for all" Bernie.
There's two kinds of people who want to dismantle the ACA; those who are ideologically opposed to government programs, and those who aren't but think then subsidy and mandate approach simply doesn't work.
The former aren't even remotely swing voters, and the latter seems to be a good fit for Sanders, who, after all, is one of them.
Bernie has two problems, he is too old (by 2020), and he is too extreme for most Democrats. Sure 45% of Democrats really really really love Bernie, but to win president you need a bit more than that, and to pull a few swing voters in. I cannot imagine the people who want to dismantle the ACA would swing over and vote for "free education for all" Bernie.