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to grow senile; to be childish with age
to be mentally slow; to be befuddled
to play stupid; to say something stupid (oft. on purpose; esp. as part of a comedic routine)
父は八十を過ぎてもまったく耄けていない。
My father shows no sign of senility even past eighty.
耄けてしまう前に家族の記録を残しておきたい。
I want to leave a family record before I lose my faculties.
耄けた祖父の世話を家族で交代して行っている。
The family takes turns caring for our senile grandfather.