The surviving children of a duplicate Earth, kept young for three hundred years by a failed life-prolongation experiment that killed every adult and still kills each child on reaching puberty.
The Onlies appear in the Original Series episode Miri. In 2266 the Enterprise answers an old distress call and finds a planet that looks exactly like Earth, its cities in ruins. Three centuries earlier its scientists had engineered a virus meant to extend life. In children it slows ageing to about a month for every hundred years; in adults it brings sores, madness and death.
The children who remained call themselves the Onlies and refer to adults as Grups. They live wild, playing games and fearing the grown-ups, and each of them falls ill as soon as adolescence finally arrives.
The landing party is infected within hours. Miri, a girl on the edge of puberty, grows attached to Kirk, while the boy Jahn leads the others in stealing the communicators. Kirk wins them over and McCoy tests a vaccine on himself. Teachers and doctors are sent to care for them.