Prime Minister Justin Trudeau says the incoming Quebec government's plan to raise the legal age for smoking cannabis to 21 could leave an opening for organized crime.

Speaking to reporters this morning on the last day of a trip to Armenia, Trudeau said increasing the legal age could undermine one of the federal law's key aims — eliminating the black market.

Trudeau said he hopes to discuss the matter further with the province's premier-designate, François Legault, whose party was elected Oct. 1.

The federal law sets the minimum age at 18 but leaves it open to provinces and territories to establish their own rules.

In all provinces and territories, the legal age will be 18 or 19 when cannabis becomes legal next Wednesday.

But Legault has promised to raise the age to 21 from 18 in Quebec. Trudeau said he personally agrees with a legal age of 18.