The rigged nominee?
ROFL. Wow, you're full of laugh lines today. I'd have to see some convincing evidence that you'd have voted for Biden over Trump.
Age is a concern. The Presidency is physically and mentally demanding and you can see that in photos taken early in a President's term compared to later. Trump is right up there and exhibits signs of dementia and insanity, which Sanders and Biden don't. There are plenty of younger candidates to choose from as running mates if either of the "old guys" wins the nomination, so It's not a major concern at the top of the list. Once the campaign gets going, and some of the younger candidates get more name recognition and their positions better known, their numbers against Trump will get up there in the lofty heights with Biden and Sanders.
There are some sharp young candidates in the Democratic field that will get a lot of attention over time. Eric Swalwell, a congressman from California, is one of them. I've met him personally on two different occasions, and had an opportunity to discuss a couple of issues. He's a "people's representative," not a partisan hack.
It looks like there might be a couple of Republican primary challengers as well. Well, we already know William Weld is in there. He's no spring chicken, nor is Mitt Romney who is gearing up for a primary challenge to Orange Hair. So youth is on the side of the Democrats.