The child, who belonged to a nomadic tribe, was kidnapped on January 10 and held captive for a week before she was murdered. The body was dumped in a forest area and was recovered on January 17.
The police said the accused, who was initially thought to be 15 years old, has confessed to the crime. But after the investigators received contradictory documents about his date of birth, the medical board was set up on the orders of the Jammu and Kashmir High Court.
The report, a copy of which is with NDTV, said physical, dental and radiological examinations have pegged the age of the accused above 19 years.
According to officials of the Crime Branch, a retired revenue officer -- 60-year-old Sanji Ram - engineered the rape and murder of the child to drive out Muslim nomad families from Jammu's Rasana village. He had allegedly "motivated" Deepak Khajuria, a Special Police Officer and the 19-year-old main accused in the case.
Four policemen arrested in the case include two Special Police Officers. Two others - one of them a sub-Inspector - are accused of destroying evidence. The investigators say they had washed the blood-soaked clothes of child before sending the evidence to forensic laboratory.
The investigation by the state police has met with stiff resistance from the Hindu Ekta Manch and the state's BJP ministers have demanded that the case be handed over to the Central Bureau of Investigation.
The nomad families allege the Crime Branch has given in to protests and political pressure and deliberately avoided the arrest of Mr Ram. "Why has the Crime Branch not arrested Sanji Ram when they say he's the mastermind? All the protests are taking place to shield him. This shows the direction where the probe is headed," said Talib Hussain, a nomad activist.