Winnipeg radio station CJOB reports the boy, who immigrated from Sudan with his family, was only 17 when he was arrested in 2008. But immigration officials mistakenly registered his birthday as being in March 1990 instead of August.
The man's relatives say they knew about the mistake, but didn't bother to correct it because the difference was only a few months.
The teen was charged days before his actual birthday after a group of young men with unloaded BB guns approached a couple on the street.
But with immigration documents and a driver's licence saying he was already 18, the teen's lawyer, Adam Masiowski, says there was little his client could do.
"One of the first things that happens in his statement is he says he is only 17, 'I'm not 18,' and there is no police officer that looks behind that," Masiowski told CJOB.
While his family searched for documents to prove he was only 17, the case proceeded as if the accused were an adult and he was convicted.
He faced a mandatory sentence of four years, but immigration corrected his age before the sentencing hearing.
Masiowski has asked for a mistrial. Accused under the age of 18 are dealt with under the Youth Criminal Justice Act. People charged under the act can't be named.
A bail hearing has now been set for next week.
Carlos Vialard works with immigrants trying to settle in Winnipeg and he said mistakes in documentation do happen.
"The information like this can be incorrect much more often than one would expect," he said. "I would say maybe three to five per cent of the documents might be incorrect in some way."
The young man's cousin said the family hopes the Crown simply dismisses the case.
"He grew up in a war zone," the cousin said.
"Like everyone else, they came for a better life. Their parents brought them here so that their kids could have a better life than they had, but unfortunately for him it didn't work out that way so far."