Green’s Dictionary of Slang

a dirty old man; poss. an actual or alleged child molester but usu. any older man who makes obvious his preference for women younger than he might be expected to pursue.

1957		J. Braine Room at the Top (1959) 133: ‘I’m very old. I’m twenty-five. A genuine DOM.’ ‘What’s a DOM?’ [...] ‘Dirty Old Man.’.
1972		B. Rodgers Queens’ Vernacular 64: dirty old man [DOM] 1. one who leches after bodies years younger than himself. 2. auntie who chases prospective partners away from each other by his presence [...] 3. (les sl) any heterosexual man, but especially one who keeps trying to bed down with a lesbian.
1982		Barr & York Sloane Ranger Hbk 158: DOM n. Dirty old man.
1986–7		Maledicta IX 155: The CYTs (cute young things) are picked up in penny (and more recently electronic games) arades [...] by DOMs (dirty old men).
2024		J. Meades Empty Wigs (t/s) 575: [C]lassic D O M ... can’t begin to think how old... M T F ...