He doesn't.
Do you guys not know old people? He's an old people.
My father's his age and is a shadow of who he was just ten years ago. Fuck, I'm twenty five years younger and I'm no where near where I was intellectually ten years ago. There's a reason chess champions fall away as they age, time is a bitch.
And dementia is a specific thing, you should have to actually prove that. Medical boards have said very publicly he shows no signs of the disease and anyone diagnosing him from a distance is an imbecile.