Turkana Boy was fully grown to the size of modern man but with a much smaller brain. This specimen was a child by every definition. This means he had the full power of a man but none of the sophistication, experience and cautious restraint. In addition, he lacked the council of wizened elders. He bore the personality of a savage delinquent. All he could do was rape and pillage his way across the environment and leave his mark on every other hominid, or kill them.
"Estimates of the individual's age at death range from 7 to 11 years old."https://en.wikipedia.org/wiki/Turkana_Boy
https://en.wikipedia.org/wiki/Turkana_Boy