You underestimate what he has been all of his life. He is capable of delivering highly technical and precisely scholarly presentations with no notes, completely off the cuff. He is absolutely brilliant and always has been.
A degradation of what he has been is still more than almost anyone now living can deliver at the peak of their powers.
Neither of us knows, but while I see stark evidence of old age: weight loss, blind in one eye, etc., I don’t see any signs of mental decline.
As far as remote diagnosis, I always noticed the dark shadows around his eyes, even years ago. I thought that was a sign of heart disease. I am not a doctor.