Canada is set to legalise marijuana for recreational use by July next year.
Such use is already allowed in the US states of Colorado, Washington, Oregon and Alaska as well as California, Massachusetts, Maine and Nevada.
But Canada would become the first G7 industrialised nation to do so nationwide.
Marijuana for medical purposes is already legal but annual sales of recreational marijuana could be as high as £5.2bn.
The Liberal government in Ottawa has put forward legislation to end the prohibition of pot, and would let adults possess 30g of the drug in public.
It set the legal age at 18, but said it would be up to each province to decide if it should be higher and also how the drug is distributed and sold.
Public health experts have mostly backed a minimum age of 21 as the brain continues to develop to about 25, according to a government-backed panel.
Prime Minister Justin Trudeau believes the new measures would restrict youngsters' access to marijuana and reduce drug-related crime.
The legislation also imposes tough new penalties of up to 14 years in prison for people giving or selling marijuana to children.
"It's too easy for our kids to get marijuana. We're going to change that," Mr Trudeau said.
However, youths who are found in possession of up to 5g of the drug would not be criminally prosecuted.
Only cannabis grown by a federally licensed producer would be available for sale, though Canadians could grow up to four plants at home.
Dried and fresh pot and cannabis oil will be available first, with edible products becoming available later.
But Christina Grant, a professor of paediatrics, fears usage among the young will go up because concerns will lessen about its safety.
She said: "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.
"And there is the risk of developing psychosis if you start using cannabis as a teenager. The more you use and the younger you start, you have up to four times the risk of developing some kind of psychotic illness."