TORONTO — The most controversial thing about Canada’s move to legalize marijuana nationwide may be setting the minimum age for use at 18 — three years lower than in U.S. states that have embraced legalization — a move that is being closely watched across the continent.

Advocates for the measure, expected to pass Parliament next year, say putting the limit at 21 would encourage a black market and drive youths into the hands of criminals.

“Taking this business away from them I think is an obligation,” said former Toronto Police Chief Bill Blair, parliamentary secretary to Canada’s justice minister and the man in charge of Prime Minister Justin Trudeau’s plan to legalize.

The task force that drafted the measure reported that experts said “that setting the minimum age too high risked preserving the illicit market, particularly since the highest rates of use are in the 18 to 24 age range.”

But health experts are worried that the provision will encourage use of a substance that can have long-term consequences on still-maturing brains.

“Our recommendation is still to postpone as old as possible, ideally after 25,” said Dr. Granger Avery, president of Canadian Medical Association, which proposed setting the age at 21 only after it became clear that the government wanted it at 18.

Legalization will inevitably lead more young people to smoke marijuana in the mistaken belief that it isn’t harmful, said Christina Grant, a professor of pediatrics at McMaster University in Ontario. “One in seven youths who have used cannabis will develop an addiction to cannabis and that impacts your life, schooling, job prospects, social and emotional relationships,” she said.

The legislation introduced last month would make Canada the second country to have nationwide legalization, after Uruguay, which also set the minimum age at 18. While eight U.S. states and Washington D.C. have legalized marijuana, users there must be at least 21.

Colorado State Rep. Jonathan Singer, whose state became the first to legalize recreational marijuana in 2012, said it too should lower the age to 18 in order to stamp out the lingering black market.

“If you are old enough to go to war then you should be old enough to be trusted to use a recreational substance,” Singer said.

U.S. teens have long crossed the border into the province of Quebec, where the drinking age is 18, and some say a lower recreational marijuana age allowance in Canada could mean an influx of pot tourism among young Americans.

Canadian youth already have higher rates of cannabis use than their peers worldwide, with 21 percent of those aged 15-19 reporting they consumed cannabis and 30 percent of those aged 20-24, according to government figures.

The Canadian legislation would give each of the 10 provinces power to set the minimum age, with at least Quebec, Alberta and Manitoba likely to choose the younger option of 18 to match the drinking age. The drinking age is 19 in the other provinces. Anyone caught selling or providing pot to someone under the age of 18 could face up to 14 years in prison.

Federal Health Minister Jane Philpott has said “no product is without risk” and noted tobacco and alcohol are legal although both pose serious health risks.

“Just because a product is legal it does not mean it is advisable or recommended to use that product,” Philpott said.

David Hammond, a professor in the School of Public Health and Health Systems at University of Waterloo, said the government will have to do more to educate young people on the health risks. Anyone arguing for pot age restrictions until 25 should be doing the same for alcohol and tobacco, he said.

“It will be hard to argue that legalizing won’t normalize it to some extent,” he said. “You are loosening the restrictions.”