Who said this?

"The man who is not a socialist at 20 has no heart, but if he is still a socialist at 40 he has no head."

I recall reading this saying some three decades ago, and for a long time I had thought this was one of Winston Churchill's long list of quotable aphorisms. However, in this article on John Howard's appeal to Australian youth, Caroline Overington ascribes it to former French Prime Minister Aristide Briand, who had once been a socialist himself before being kicked out of the party. However, this website suggests that Briand may have borrowed the saying from an earlier French statesman, François Guizot, who had phrased it this way: "Not to be a republican at twenty is proof of want of heart; to be one at thirty is proof of want of head." I suppose it's still possible Churchill could have said some variant of this at some point, but this site indicates the lack of documented evidence for him having done so publicly.