The high court unanimously agreed with a lower-court judge who declared the 15-year-old's police statement inadmissible.

Justice Morris Fish, writing for four of the seven Supreme Court judges, placed the onus on the police to determine a person's ability to understand their instructions.

"Police officers, in determining the appropriate language to use in explaining a young person's rights, must . . . make a reasonable effort to become aware of significant factors . . . such as learning disabilities and previous experience with the criminal justice system," Fish wrote.

A statement should not be admitted if a trial judge is not satisfied a young person understood their right to consult counsel or to have a parent present, or the consequences of waiving those rights, he wrote.

"The trial judge was . . . bound to find the statements inadmissible if she was not satisfied beyond a reasonable doubt that the young person's rights were explained to him in language appropriate to his understanding or if she had a reasonable doubt whether the young person understood his right to counsel and therefore could validly waive it," he said.

"It is clear from her reasons that she was left with a reasonable doubt in both respects."

The boy, who was charged with dangerous driving causing bodily harm, was read his rights, said he understood them and signed a waiver-of-rights form.

But the youth court judge ruled his videotaped statement inadmissible, saying prosecutors failed to prove "beyond a reasonable doubt" that the statement was voluntary and that Youth Justice Act requirements had been met.

The Court of Appeal allowed the appeal, set aside the verdict of acquittal and ordered a new trial.

The accused took that decision to the Supreme Court and won.

Four of the Supreme Court justices agreed with the "reasonable doubt" standard set by the lower-court judge, but three others said that, while it doesn't change the acquittal, holding the burden of proof in such cases to a "balance of probabilities" was standard enough.

The charge against the boy was laid after an Aug. 8, 2004, accident that nearly killed a 24-year-old man who was walking home from a friend's bachelor party in Dartmouth.

The man spent two weeks in a coma and wasn't expected to live.

The accused teen gave a 90-minute partial confession to police that day. He originally claimed he wasn't the driver, but later admitted he was at the wheel.

The boy was sentenced to a year in a youth detention facility in September 2005 for a host of other motor vehicle-related offences.

During sentencing at that time, the court heard the boy had racked up 39 previous convictions since he began stealing cars as a 12-year-old.