I'm not sure it is a joke, Wilfried; perhaps a witty saying. From "Nice Guys Finish Seventh: False Phrases, Spurious Sayings, and Familiar Misquotations" by Ralph Keyes, 1992:

"An orphan quote [unattributed quote in search of a home] sometimes attributed to Georges Clemenceau is: Any man who is not a socialist at age 20 has no heart. Any man who is still a socialist at age 40 has no head.

The most likely reason is that Bennet Cerf once reported Clemenceau's response to a visitor's alarm about his son being a communist: "If he had not become a Communist at 22, I would have disowned him.   If he is still a Communist at 30, I will do it then."

George Seldes later quoted Lloyd George as having said: "A young man who isn't a socialist hasn't got a heart; an old man who is a socialist hasn't got a head."

The earliest known version of this observation is attributed to mid-nineteenth century historian and statesman François Guizot: "Not to be a republican at 20 is proof of want of heart;   to be one at 30 is proof of want of head."

Variations on this theme were later attributed to Disraeli, Shaw, Churchill, and Bertrand Russell. (I misquoted Churchill to this effect for years.)"