Jahn was the leader of the Onlies, a band of feral children on the Onlies' homeworld. These children had been alone for over three hundred years, since an artificial disease, the life prolongation complex, killed all of their parents. When the USS Enterprise landing party encountered him in 2266, he was over three hundred years.
When the USS Enterprise visited in 2266, 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. He was cured of his 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 got the fatal form of the life prolongation complex.
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 the other member of his "crew," though the youngest, Pal, is beamed safely aboard the Enterprise.