I have been getting a lot of this in recent years, regarding my father. I never know what to say, really (see: his reaction). However, if an older man is the one making the comment, I will always say “oh, well he’s good-looking so thank you!” Because so very many of old guys are petrified with the idea of ever complimenting another man’s appearance. And I like to see their faces when I say my dad is good-looking and also subtly intimate that they think as much.
And whatevz, my dad IS good-looking. Have you seen that moustache? It is robust. Hearty like Campbell’s soup.