Canada legalizes marijuana but it is not legal to bring it into the U.S.

Marijuana may be legal in some states in the United States, but it is federally prohibited. This means it is illegal to bring marijuana bought in Canada across the border into the United States.
This is the case even if you are flying into a state where it is legal, such as Colorado.  Crossing international borders with cannabis is illegal.
Flying with pot from Vancouver to Montreal, on the other hand, is perfectly legal so long as you meet the age requirement and carry no more than 30 grams of cannabis.
“As long as the flight is domestic, people are allowed to bring a certain quantity for their personal use,” said Canadian Federal Transport Minister Marc Garneau in a statement.

To use recreational marijuana in Canada you must be at least 18, although some provinces set their minimum age at 19. The production, distribution or sale of cannabis products is an offense for minors.

However, the minimum age for recreational possession of marijuana in US states where it is legal is 21, adding to the murkiness — and danger — of traveling internationally with a substance prohibited by US federal law.