Green’s Dictionary of Slang

an older man whose sexual tastes (whether or not fulfilled) err towards much younger lovers, esp. when under the legal age of consent.

1940		F. Sargeson ‘A Man and his Wife’ in A Man And His Wife (1944) 75: We all chipped in to say he was a dirty old man, and it was no wonder his missis had kicked him out.
1957		J. Braine Room at the Top (1959) 133: ‘What’s a DOM?’ [...] ‘Dirty Old Man.’.
1959		K. Waterhouse Billy Liar (1962) 39: He’s got a mucky book under his jersey! Coarrr! Dirty old man!
1960		J. Iggulden Storms of Summer 34: Christ! You make me sound like a dirty old man, the way you say that.
1967		N. Dunn Poor Cow 81: Here I am touching up this dirty old man for a couple of quid.
1970		‘Troy Conway’ Cunning Linguist (1973) 01: ‘That old lecher. Look at him. The prototype of a dirty old man.’‘An ass-pincher if ever I saw one,’ I agreed.
1984		Benjamin & Pearl Limericks Down Under 39: A dirty old man of Tarcutta / Was an unregenerate rutter.
1985		K. Vacha Quiet Fire 70: I don’t mean to be a lush or a dirty old man in a bar.
1998		D. Senna From Caucasia, With Love (2000) 66: There are perverts, crazies, dirty old men, and they want little girls like you.
1999		Guardian Guide 19-25 June 4: It also has the Dirty Old Man demographic by the short hairs.
2006		www.downloadingpornwithdavo.com 🌐 The Dirty Old Man. He’s a public pervert! And truly proud of it! Watch him fuck young women!