Prosecutors had argued that the man, who was arrested in 2006, knowingly took part in military exercises and firearms training at the camp as part of a conspiracy.
The defence said that the group's plot to attack Canadian targets was a "jihadi fantasy" which would have been impossible to carry out.
But in his ruling, Sproat said: "It might well have been said prior to September 11, 2001, that a plan to kill thousands and destroy landmark buildings in lower Manhattan and Washington had no possibility of implementation."
Prosecutors have alleged that the arrested group members, known as the "Toronto 18", had plans to bomb nuclear power plants and a building housing Canada's spy service.
The group also planned to storm the country's parliament and behead the prime minister, prosecutors say.
Seven of those arrested have since had their charges either withdrawn or stayed, while 10 adults are yet to face trial. The legal minor was the first to go before a court.
Sproat rejected defence arguments that camps were for religious introspection or recreation.
"Apparently benign activities may be used to identify and indoctrinate recruits," he said.
There was clear evidence that the young man listened to the plot's ringleader and had aimed to please him, the judge said.
Mitchell Chernovsky, the youth's defence lawyer, said he was not sure how long his client's sentence would be. The young man faces a maximum 10-year sentence.
Wesley Wark, a University of Toronto professor and an expert on national security, said the guilty verdict shows the tough nature of Canada's anti-terrorism laws.
"You can be convicted for terrorism even if nothing particular happens as a result of a plot, even if the plot looks amateurish, even if you did not fully know the details of the conspiracy you were a part of," he said.
Evidence for the prosecution was collected by Mubin Shaikh, a former Canadian army cadet, who spied on the alleged cell members before their arrests.
Shaikh received about $300,000 for infiltrating the group, but Sproat noted that the defence did not claim that the payment influenced Shaikh's evidence.