I had multiple grandparents at different states of decline from dementia during the fateful debate. Those weren't his usual stutter nor a simple misspeaking, that was his brain being unable to connect the partial fragments of thoughts and voice them. Aka dementia.

Trump's also showing clear cognitive decline, but he's also much more obviously very hopped up on medications for all of his public speaking engagements. There's a ~4 week cycle of him having a poorly covered up bruise from an IV in his hand, about a week of him hidden from the public eye, then about 2 weeks of him doing noticably better then he starts declining again for the last week before the next dose during which he gets hidden from the public eye again. They schedule all of his biggest public engagements during that 2 week window

Biden's final debate performance was pretty awful. Seemed lost a couple times. Nothing like the guy who had debated Paul Ryan years before. It was so bad that he had to drop out.

I am not comparing him to Trump, Trump is clearly spiraling out of control. The problem is he had proof of problems and they hid them. No one knows the degree, so it is being used by anyone who can make money from it to postulate that he was basically either unaware of what was happening around him, or being sharper than most people in their 20s, neither was true. At some point we may learn what was happening.