At Trump's age he has what is politely known as a "Senior Moment" when commonly known names or information momentarily (sometimes for days) escape him. It is an early sigh of Alzheimer's that tends to get worse rather quickly and more often.
Trump's doctors would be remiss if they weren't clocking this decline in Trump's cognitively closely.
Of course, Trump would adamantly deny such a thing even if and when the truth hits him smack in the face.
Intervention is sometimes required, depending on the circumstances. The Senate probably hasn't reached the 25th Amendment stage yet . . . but considering the rapid progressive nature of the disease, it won't be long.
My guess is that his staff will keep the disease hidden from the public for as long as possible.
Suspicious enough for him to quit his job over it which means he's probably guilty.