You will recognize, my boy, the first sign of age: it is when you go out into the streets of London and realize for the first time how young the policemen look.
Source, in full
Quoted in C. R. D. Pulling They Were Singing (1952) ch. 7. Cited in J. M. and M. J. Cohen (eds.) The Penguin Dictionary of Modern Quotations (1971) p. 97 and in The Oxford Dictionary of Quotations, 3rd ed. (1980 [1979]) p. 250