I really think if Bernie was ten years younger, then he would have a stronger chance of getting the nomination. He gets people going and believing in him. However no one really wants a really old president in America. Sure Biden is out there as well not much younger than Bernie but I am not sure people really want him either.
I think the principle problem Bernie Sanders faces is he announced his candidacy (and his membership in the Democratic Party) on the premise that there was no fucking way Hillary Clinton could run unopposed. Now - you can feel any way you want about that. Personally I'm all about it. But he's also saying "dear Democratic power structure: up yours. Sincerely, Bernie Sanders." And that's why they did shit like fuck with the primary rules to keep him out. And over the past four years, the DNC has shown little-to-no contrition for this.