“I had entered the communist children's movement, an organisation called the Young Pioneers of America, in 1930 in New York City; I was only nine years of age. And I'd gone through the entire '30s as a—Stalinist—initially, and then increasingly as someone who was more and more sympathetic to Trotskyism. And by 1939, after having seen Hitler rise to power, the Austrian workers revolt of 1934 (an almost completely forgotten episode in labour history), the Spanish revolution by which I mean the so-called Spanish civil war—I finally became utterly disillusioned with Stalinism, and drifted increasingly toward Trotskyism. And by 1945, I, finally, also became disillusioned with Trotskyism; and I would say, now, increasingly with Marxism and Leninism.”

Television Interview, Aeschbacher April 4, 2008, Swiss Television SF1

Source: The Rose Petal Beach

No Rich Child Left Behind, 2013

Letter to John Spencer (19 February 1900), quoted in John Wilson, C.B.: A Life of Sir Henry Campbell-Bannerman (London: Constable, 1973), p. 326
Leader of the Opposition

Die Österreicher haben das Kunststück fertiggebracht, aus Beethoven einen Österreicher und aus Hitler einen Deutschen zu machen.
As quoted in DER SPIEGEL (16 May 1994) http://www.spiegel.de/spiegel/print/d-9281967.html?name=Sp%26auml%3Bte+Heimkehr)

Original preface to Animal Farm; as published in George Orwell: Some Materials for a Bibliography (1953) by Ian R. Willison
Context: I have never visited Russia and my knowledge of it consists only of what can be learned by reading books and newspapers. Even if I had the power, I would not wish to interfere in Soviet domestic affairs: I would not condemn Stalin and his associates merely for their barbaric and undemocratic methods. It is quite possible that, even with the best intentions, they could not have acted otherwise under the conditions prevailing there.
But on the other hand it was of the utmost importance to me that people in western Europe should see the Soviet regime for what it really was. Since 1930 I had seen little evidence that the USSR was progressing towards anything that one could truly call Socialism. On the contrary, I was struck by clear signs of its transformation into a hierarchical society, in which the rulers have no more reason to give up their power than any other ruling class. Moreover, the workers and intelligentsia in a country like England cannot understand that the USSR of today is altogether different from what it was in 1917. It is partly that they do not want to understand (i. e. they want to believe that, somewhere, a really Socialist country does actually exist), and partly that, being accustomed to comparative freedom and moderation in public life, totalitarianism is completely incomprehensible to them.

Lt. Col. Eugene K. Bird on the death of Hess, to a Deutsche Presse-Agentur reporter, as quoted in "Former governor of Spandau Prison dies in Berlin" in Expatica (7 November 2005)

So I stabbed him.
E=MO² (1985)

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