Old Man Yaoi

Old Man Yaoi is a specific brand of Yaoi- Japanese gay erotic/romantic manga, in which the two characters in love are old. More often people use the term to describe fiction relationships containing old men that are from other types of media.

Person 1: "What are you reading?"
Person 2: "Oh, it's Hilson Mpreg Fanfiction. They're my favorite Old Man Yaoi."