Biden was not too old. He is saddled with infirmities that present themselves with age. Not everyone is afflicted equally or at the same age. My father died a few years ago at 97 and in full on dementia. My mother will be 101 in a few months and she is sharp as a tack.
I don't want to have an argument, and I actually agree in the abstract with everything you said here.
But about ten years ago, my own Dad (who had then just turned 70) said that someone his age had no business in the nuclear era being President of the United States. He said it's not that you're even not sharp so much as it is that you MUST be able to be roused out of a sleep at 3am and make life and death decisions with a clear head, and if you have a Cuban Missile crisis that might be a two-week thing.
Every single person over the age of 70 - and this is from my med school books - has SOME level of dementia. No, most people are not where Biden appeared to be at 80, but Biden also had a long history of family trauma, speech impediment, and aneurysms that made his obviously worse.
And given what was covered up or "don't believe what you see," it's fair to wonder what else might be true in those medical records, too.
But I think the Democrats DO need to listen to David Hogg in the sense they now have these career guys who are hanging around too long and running the risk of losing seats to the other party.
What if Pete Wilson or Arnold Schwarzenegger had been the governor of California when Feinstein died?