"Old Man" was Benjamin Sisko's nickname for the Curzon host of the Dax symbiont, which he continued to use with the symbiont's later hosts. (DS9: "Emissary", "Dax", "Move Along Home")

Benjamin Sisko occasionally named himself "Old Man" when he talked to his son Jake. (DS9: "The Nagus")

When Jadzia decided to stay on Meridian, which would have meant the two would not see each other for sixty years, Jadzia joked that she could call Sisko "old man" when they would see each other again. (DS9: "Meridian")

Sisko also called Yedrin Dax "Old Man" when they met in 2373. (DS9: "Children of Time")

The hosts of Dax
Lela • Tobin • Emony • Audrid • Torias • Joran • Curzon • Jadzia • Ezri • Yedrin • Verad