mental disorder: senile dementia

Literally, dementia in old age, this term probably most often refers to the dementia of Alzheimer's disease.

Also: senility (Although this word is used in popular culture as synonym for dementia it just means old age, perhaps reflecting the myth that age universally brings decline in cognitive function.)

Books and Other Media:
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Is Anybody There? Bill Milner, Anne-Marie Duff, Michael Caine 2008

The Descendants George Clooney, Shailene Woodley, Judy Greer 2011

Pinky Jeanne Crain, Ethel Barrymore, Ethel Waters 1949

Undercurrent Katharine Hepburn, Robert Taylor, Robert Mitchum 1946