Meaning of the word "every man is either a fool or physician by forty" in English

What does "every man is either a fool or physician by forty" mean in English? Explore the meaning, pronunciation, and specific usage of this word with Lingoland.

every man is either a fool or physician by forty

US /ˈevri mæn ɪz ˈaɪðər ə fuːl ɔːr fɪˈzɪʃən baɪ ˈfɔːrti/
UK /ˈevri mæn ɪz ˈaɪðər ə fuːl ɔː fɪˈzɪʃən baɪ ˈfɔːti/
"every man is either a fool or physician by forty" picture

Idiom

by the age of forty, a person should have enough common sense and experience to understand their own health and body better than anyone else

Example:
He started eating better and exercising more, proving that every man is either a fool or physician by forty.
You should know what makes you feel ill by now; every man is either a fool or physician by forty.