Jahn was the leader of the Onlies, a band of feral children on Earth Two. These children had been alone for over three hundred years, since an artificial virus, created by a life prolongation project, killed all of their parents. When a landing party from the USS Enterprise encountered Jahn in 2266, he was over three hundred years old.
During the Enterprise's visit, Jahn led the children against the Enterprise landing party, first stealing their communicators and then kidnapping Janice Rand. Eventually, Captain Kirk persuaded Miri to lead him to the other Onlies; there, he managed to persuade them to return Rand and the communicators. Even though Jahn didn't yet develop the disease, he was cured of the infection by a vaccine developed by Dr. McCoy.
Jahn kept the children together, organized them, formulated their tactics, and led them in other activities, such as foolies. He was far less enthusiastic than Miri about the presence of the landing party; like all of the Onlies, he knew what happened when adults caught the fatal form of the aforementioned virus.
Jahn was portrayed by Michael J. Pollard.
In the first draft script of "Miri", Jahn had more of a relationship with Miri than is depicted on screen. In their distorted reality, Jahn was similar to the titular character from Peter Pan, while Miri represented the character of Wendy from that story. (The Star Trek Compendium, 4th ed., p. 42)
When first introduced in the final draft script of "Miri", Jahn was described thus; "Dominating the scene, in the center, perched on an elaborate kid-sized bandstand, complete with the equipment for a Beatles-type trio [...] is a boy just a little younger than Miri, named Jahn. He has chosen to be dressed as a sort of Beatle, complete with a wig, the price tag still dangling, which is somewhat askew. Jahn isn't evil [....] He smiles a lot, he likes to laugh. He tells himself he's living in the best of all possible worlds. He almost believes it – after all, he's been telling himself this for 300 years. But there is a forlornness under the surface – it has its own way of coming out."
Even in the final draft of the "Miri" teleplay, there was more between Jahn and Miri than in the final version of the episode, with the script stating about them, "There is something between these two – 300 years ago, if everything hadn't gone wrong, they would have been childhood sweethearts, we feel. Somewhere deep inside of them, they know this, too."
In the novel The Cry of the Onlies, Jahn rebels against the Federation educators left on the planet to help the Onlies into adulthood; he starts a fight by grabbing a security guard's phaser and several of the Onlies are killed, including Miri. Jahn and two other Onlies then hijack a Starfleet vessel in orbit, equipped with an experimental cloaking device developed by Flint. After a short rampage through the border region between the Federation and the Klingon Empire, the ship is cornered by the Enterprise and destroyed, killing Jahn and another member of his "crew," though the youngest crewmember, Pal (β), is beamed safely aboard the Enterprise.