Stephen Fray was quickly led out of the courtroom as relatives and friends cried. His attorney, George Thomas, said he would appeal the ruling.
"I can't believe it," he said. "It's very harsh. No one took into consideration that this man is sick."
Authorities had described Fray as "mentally challenged" and said his motive remained unclear. But a judge ruled he was fit to stand trial after a government psychiatrist determined he was able to understand the charges against him.
Fray was found guilty last month on charges that included robbery and assault.
Police said Fray pushed his way into a CanJet Airlines plane on April 19 and fired his father's licensed .38-caliber revolver into the air. He stole money from some of the 167 passengers aboard and demanded to be flown off the island, authorities said.
He was captured after a six-hour standoff with police.
Public prosecution director Paula Llewellyn said she was satisfied with the outcome and confident with the testimony of 22 witnesses.
"It's unfortunate that this young man found himself in such a situation, but what is important is that citizens both local and international saw that the case was handled in an efficient manner," she said.