Transit authorities say the attack happened outside the Braid Street SkyTrain station Saturday evening.
The victim, who was waiting for a bus, apparently tried to intervene when he saw the group of teens — one boy and three girls — allegedly kicking the nearby bike lockers.
“He yelled at them to stop, ‘Hey you shouldn't be doing that.’ The boy took umbrage to that and the boy came rushing at him with a knife in his hand,” said Transit Police spokesperson Anne Drennan.
The victim was allegedly punched in the head and had his briefcase stolen before the group ran off.
Two teens were later arrested in connection with the alleged attack.
Drennan said the 14-year-old boy has already had more than 50 run-ins with the police, while a 13-year-old girl has had more than 100 run-ins.
Because of their ages, young offenders often don't get the help they need, she said.
"These kids are out there, they get more and more aggressive as time passes. They aren’t afraid to attack people, they aren't afraid to rob them,” she said.
“When police arrest them and get charges laid, it's a temporary fix. These kids know that the consequences are just not there, so when they're released on conditions, the conditions mean nothing to them.”
The 14-year-old boy from Coquitlam has been charged with assault with a weapon and theft under $5,000. He has also been charged with breaching his release conditions related to two prior robbery charges.
The teen is being held in custody until his next court appearance, scheduled for June 11.
The 13-year-old female suspect has been released with charges pending until further investigation.