The phrase "mysterious old man" is an idiomatic expression that refers to a person who is perceived as being enigmatic, secretive or mysterious due to their age and/or appearance. Full definition
Friedberger's debut may not be as brilliant, but it goes about depicting summer in New York in a similarly organic fashion, casting a city full of mysterious old men and rambunctious kids on the F train. (metacritic.com)
A real estate reporter investigates Jebediah Crone (Dayton Callie), a mysterious old man who is building a haunted house constructed from rooms where deaths have occurred. (firstshowing.net)
Her immigration status was precarious, her brothers fighting in the war, and a neighbor had dumped her at the door of a mysterious old man, a shoemaker the girl did not know, but who was the only person standing between this girl and an orphanage. (bookpage.com)