Cognitive ability alone is not the whole story, the ability to be persuasive and communicate well is also a key point. I watched the whole debate, and Biden did not communicate well at all. He started out lousy, got a bit better in the middle, but then fell off the wagon again. I think if he had finished strong it would be a lot different.
Age is a key concern because it will affect everyone. Not every 80 year old is diminished, and not everyone has that mental decline before their physical decline accellerates. But we all end up in the same place, in the end. So his age is the one concern that Biden can’t counteract with a policy change or an executive order. All he can do is show us what he has. And he simply didn’t deliver, even after all his debate prep.