My mother-in-law was a former headmistress and published short story writer and yet in her closing years suffered extreme dementia - not recognising family members, spitting out food and even violent behavior. Very distressing.

At least 2 former UK Prime Ministers suffered dementia - Harold Wilson and Margaret Thatcher, who were both formidable politicians in their pomp at one time, particularly Thatcher. I disagreed with almost all Thatcher stood for but she was certainly very able (too able IMO!) unlike the current US President, who shows signs of dementia while in office. Wilson had the good grace to resign when he felt his mental powers were declining.