He won the popular vote by a smaller percentage of votes cast than Hillary and the Dem candidate winning the sub-45yo vote is pretty much a given. In most elections, it would be by much more.
As for the 65+ group, a lot of fascists and people who don't consider fascism more of a deal breaker than a (D) behind a name are aging into that group, perhaps as many as are dying off.
And no, I'm not assuming anything. I'm pointing out that there's a big risk of the pro-democracy candidate losing if he and the rest of the DNC don't stop thinking that "Not Trump" is enough and start listening to what the public wants.