This NYT article from 2012 addresses this point: "Can You Call a 9-Year-Old a Psychopath?", http://www.nytimes.com/2012/05/13/magazine/can-you-call-a-9-...

The article's answer is basically yes, but it is not easy. Yes, many of the typical criteria used to diagnose psychopathy are normal traits in children at certain ages. But one of the counterpoints is that what are the chances that people suddenly become psychopaths in adulthood? It's likely they exhibit some diagnosable traits in childhood.