Born around 1950, Jahn was only a child when his parents and the rest of the adult population began to die from an artificial disease created by a failed life prolongation complex. Over the next three hundred years, Jahn led the surviving children, organizing them and keeping them together.
When the USS Enterprise visited the planet in 2266, Jahn and most of the other children were wary of "Grups" returning, and began to attack the landing party, by stealing their communicators and then kidnapping Yeoman Janice Rand. With the help of Miri, Jahn and the Onlies began to trust Captain James T. Kirk and agreed to the return of the communicators and Yeoman Rand so that a cure for the disease could be found. (TOS episode & novelization: Miri)
Following the departure of the Enterprise, Jahn and the other Onlies were cared for and educated by Federation personnel when the planet became a Federation protectorate. Jahn, however, repeatedly rebelled against their authority and eventually, together with a younger girl named Rhea, managed to sneak onto a Federation supply vessel, the USS Sparrow and take control of the ship, with which he planned to leave the planet. When he tried to persuade more Onlies to join him in leaving, a firefight with Federation personnel ensued, in which Miri, among others, was killed. All this violence and death caused the runaways to become unstable and start attacking ships. (TOS novel: The Cry of the Onlies, ST reference: Star Charts)