ぼける
JLPT N1 · Verbぼける(bokeru)声调 2
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)
年を取ると、頭がぼけてしまう。— As you get older, your mind starts to fade.