On being told his son had joined the Communist Party, as quoted in Try and Stop Me (1944) by Bennet Cerf
A statement similar in theme has also been attributed to Clemenceau:
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.
As quoted in "Nice Guys Finish Seventh" : False Phrases, Spurious Sayings, and Familiar Misquotations (1992) by Ralph Keyes.
W. Gurney Benham in A Book of Quotations (1948) cites a statement by François Guizot as the earliest known expression of this general idea, stating that Clemenceau merely adapted the saying substituting socialiste for republicain:
N'être pas républicain à vingt ans est preuve d'un manque de cœur ; l'être après trente ans est preuve d'un manque de tête.
Not to be a republican at twenty is proof of want of heart; to be one at thirty is proof of want of head.
Variations on this general idea have also been attributed or misattributed to many others, most commonly Winston Churchill, who is not known to have actually made any similar statement.
Post-Prime Ministerial