Sometime around Earth year 1960, the life prolongation complex annihilated the entire adult population of the Onlies' homeworld, leaving only the feral children who called themselves the Onlies. These children had survived without adult care for over three hundred years until the USS Enterprise found them in 2266. The children ranged in age from four to pubescent.
The children scrounged for food where they could. They knew the area where they lived extremely well, and were fond of games they called foolies. These games involved taunting the Enterprise landing party. The children occasionally became more violent, especially when they felt threatened. The events of the plague had instilled in them a fear and distrust of grups; it took a great deal of effort for James T. Kirk to convince them that he and his people meant them no harm.
These children were infected with the life prolongation complex, just as the adults had been. In the children, the complex functioned as intended, greatly increasing their lifespans. But when they reached puberty, the complex entered its fatal form, and death followed rapidly.
When the Enterprise left, the children remained on their world, in the care of a medical team. Additional specialists were dispatched from elsewhere in the Federation to care for them. It is fortunate that the Enterprise found them when it did; diminishing food supplies would have meant starvation for most of the children within a few months time. (TOS: "Miri")
The Onlies were the focus of Judy Klass' novel The Cry of the Onlies, in which it was shown that the chief doctor who was sent to gradually bring the Onlies into adulthood and educate them was instead exploiting and abusing them. The Federation was testing a prototype spaceship on the planet, and Jahn stole it and began rampaging across the galaxy after Miri's death. Kirk and company straightened out the situation and arranged for Flint to become the Onlies' new mentor and guardian.
Another take on the Onlies was in issue #3 of Marvel Comics' Star Trek: Untold Voyages, titled "Past Imperfect", which showed Jahn marauding after Miri's death, but this time he kidnapped Dr. McCoy's daughter, Joanna, because McCoy's cure had mutated into an even worse disease.