The jury at an inquest into the killing of Daniel Hegarty by British soldiers in Londonderry in 1972 found the boy "posed no risk" when he was shot.
The 15-year-old was shot during Operation Motorman on 31 July 1972.
Daniel, who was a labourer, was shot twice in the head by a soldier close to his home in Creggan. His cousin Christopher, 16, was shot in the head by the same soldier but survived.
On Friday, following a five day hearing, the jurors unanimously found that neither teenager posed a risk when they were shot.
The jury also rejected the soldiers' claims that they had shouted warnings to the two teenagers before they were shot.
They found that none of the soldiers present attempted to "approach the injured youths to either search them or provide medical assistance".
The initial inquest was held in 1973 and recorded an open verdict. A second inquest was ordered by the Attorney General in 2009 following an examination by the Historical Enquiries Team.
The report found that the RUC investigation at the time was "hopelessly inadequate and dreadful". It also said Daniel "posed no threat whatsoever" when the soldiers opened fire.
The inquest opened on Monday and heard from Daniel's sister Margaret Brady. She described how her mother continued to set a place for him at the table and call him for dinner for months after his death.
In 2007, the British government apologised to the Hegarty family after describing Daniel as a terrorist.
A Ministry of Defence document, assessing the Army's role in Northern Ireland, also incorrectly claimed the 15-year-old was armed.
The names and circumstances of the death of the victims referred to on this website comes from court documents, witness statements, official reports, and respected human rights and civil liberties organisations such as Amnesty International. Allegations of the British Army Force Research Unit collusion in the murder of UK citizens, including the murder of human rights lawyer Patrick Finucane comes from the British Police Commissioner Sir John Stevens who conducted the official enquiry into British Army Collusion.