In the 1950s, the inhabitants of the planet began experimenting with a life prolongation complex. Unfortunately, the affect of the project was an artificial disease which killed all the adults and children who were entering puberty. Before the inflicted died however, they developed blue blemishes on their skin and began to soon began acting dangerously and insanely.
Younger children were immune from the disease until they reached puberty, and during the initial outbreak, they were forced to hide from the adults until they died. Three centuries later, the children still had a deep-rooted fear of adults, which initially led them to distrust the landing party from the USS Enterprise. (TOS episode & novelization: Miri)