You do realize that age is not a perfect proxy for mental competence? A good number of people remain mentally sharp well into their 90s, while others experience rapid dementia as early as their 60s.
I’m not saying his age wouldn’t be a talking point, but I’m damn sure Bernie could express his platform with more clarity and vitality than Biden at this point. Unfortunately I dont think it’s a real possibility, but it’s stupid to act like the the actual birth date matters. It’s the signs of cognitive decline that are problematic.