Take ten years off Bernie's age; how strong a position would he be in?

[America Needs R'hllor]

Bernie Sanders knows it's too early to be discussing or even thinking about 2020.

Yeah, right. 

Trust me, these guys are already thinking about 2020.  Not just Sanders, but Warren, Booker, Gillibrand....all of them.  Not that I blame them, but "It's too early to think about that" is a convenient lie that they all tell in order to not appear too eager to move on from their current jobs.  This is especially necessary for those who are up in 2018, and have to go through the fiction of being interested in serving another complete term rather than move on to a presidential campaign a few months after being reelected.

Indeed.
And yes, he would be the clear frontrunner and I think that the narrative would add a lot to his poll numbers. Narrative matters, after all. Warren would probably be considered unlikely to run.

[America Needs R'hllor]

Another fun question: Would Biden be the frontrunner if he were five years younger?

I suspect he would be in Warren's position- at the lead in most polls, but still not a sure thing, and also thought by some as unlikely to run.