" By the way, you have to be a real shit to suggest that succumbing to old age is the same as being incompetent."
Let's both admit that it's not about Joe's age, it's about Joe's mental condition. There are people older and younger than him that ALSO suffer from diminished mental capacity, as well as a lot of folks older/younger who DON'T.
But more to the point you made in the quote (leaving out the "real shit" part, which if I had said, would have given me my 99th timer...) the sad fact is that the brain and the body rarely give out at the same time as we age. If the brain goes first, which absolutely happens as a byproduct of old age, then yes, with old age can come incompetence, mental incompetence.
But polite people don't refer to the average octogenarian who suffers from mental decline as incompetent. We treat our seniors better than that. Or we should. The mental incompetence is a symptom of diminished or lost mental capacity, not the cause of their condition.
Based on my experience, Joe's actually not doing too poorly for a person his age. Obviously I don't know his personal details that are none of my business, so I can only go by what I've seen, and it's not good.
My personal opinion is that he would be doing even better sitting on a porch and drinking lemonade. You have to recognize that everyone else in his mental and physical condition doesn't have the nuclear codes, and doesn't ostensibly make life and death decisions for those serving in the military.
The Joe from the debate, and the Joe from Clooney's fund raiser, are simply not capable of being POTUS. That cannot be up for discussion.