“Si vous n'êtes pas libéral à vingt ans, c'est que vous n'avez pas de cœur; si vous n'êtes pas conservateur à quarante ans, c'est que vous n'avez pas de cervelle.”

Original

If you're not a liberal when you're 25, you have no heart. If you're not a conservative by the time you're 35, you have no brain.

The earliest example of this quotation is found in Jules Claretie's Portraits Contemporains (1875), where the following remark is ascribed to lawyer and academic Anselme Polycarpe Batbie: "Celui qui n’est pas républicain à vingt ans fait douter de la générosité de son âme; mais celui qui, après trente ans, persévère, fait douter de la rectitude de son esprit" (English: "He who is not a republican at twenty compels one to doubt the generosity of his heart; but he who, after thirty, persists, compels one to doubt the soundness of his mind").
According to research http://www.webcitation.org/query?id=1256577474900567&url=www.geocities.com/Athens/5952/unquote.html by Mark T. Shirey, citing Nice Guys Finish Seventh: False Phrases, Spurious Sayings, and Familiar Misquotations by Ralph Keyes, 1992, this quote was first uttered by mid-nineteenth century French historian and statesman François Guizot when he observed, Not to be a republican at 20 is proof of want of heart; to be one at 30 is proof of want of head. (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.) However, this ascription is based in an entry in Benham’s Book of Quotations Proverbs and Household Words (1936): the original place where Guizot said this has not been located. This quote has been attributed variously to George Bernard Shaw, Benjamin Disraeli, Otto von Bismarck, and others.
Furthermore, the Churchill Centre http://www.winstonchurchill.org, on its Falsely Attributed Quotations http://www.winstonchurchill.org/resources/quotations/quotes-falsely-attributed page, states "there is no record of anyone hearing Churchill say this." Paul Addison of Edinburgh University is quoted as stating: "Surely Churchill can't have used the words attributed to him. He'd been a Conservative at 15 and a Liberal at 35! And would he have talked so disrespectfully of Clemmie, who is generally thought to have been a lifelong Liberal?"
Variants: Any man who is under 30, and is not a liberal, has no heart; and any man who is over 30, and is not a conservative, has no brains.
Show me a young conservative and I'll show you someone with no heart. Show me an old liberal and I'll show you someone with no brains.
If you are not a socialist by the time you are 25, you have no heart. If you are still a socialist by the time you are 35, you have no head.
Misattributed
Source: https://books.google.com/books?id=nIuaBX8moLkC&q=%22fait+douter%22#v=snippet&q=%22fait%20douter%22&f=false
Source: http://quoteinvestigator.com/2014/02/24/heart-head/