THORNBURY, Ontario — Parents and grandparents jammed the small hall of Thornbury, a sleepy ski town north of Toronto, to glean tips on how to talk to their teenagers about the potential harms of marijuana.

Held less than a week before Canada was set to legalize cannabis, the public health session had a message for parents: Marijuana would be legal for adults, but it was not safe for young people. And parents needed to instill in their children the idea that pot could be dangerous.

“It’s been proven the brain doesn’t stop growing until you are 25, and yet we’re legally selling it to people at 19,” Jenny Hanley, an addictions counselor, said as she left the meeting. “What the hell is our government thinking?”

Canada last month became the second country to make it legal for adults to buy, grow and consume small amounts of marijuana. But it also made it a crime to give it to anyone younger than 19 or 18, depending on the province, and set a penalty of up to 14 years in prison for doing so.