In this April 20, 2016, file photo, people smoke marijuana during a 4/20 cannabis culture rally in Toronto. The Canadian government, which introduced nationwide pot legalization legislation last month and expects to pass it next year, hopes to undercut the underground marijuana market with the lower age limit of 18 and not 21, an untested theory that is being closely watched around this country and in the U.S. (Mark Blinch/The Canadian Press via AP, File)

The most controversial thing about Canada's move to legalize marijuana nationwide may be setting the minimum age for use at 18. That's three years lower than in U.S. states that have embraced legalization.

Advocates for the measure say putting the limit at 21 would encourage a and drive youths into the hands of criminals.

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

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 , users there must be at least 21.

  • In this March 9, 2017, file photo, Canadian Prime Minister Justin Trudeau takes part in roundtable discussion on the future of energy with industry leaders at CERAweek in Houston, Texas. Canada is behind eight U.S. states and Washington DC in allowing recreational marijuana use but in at least one respect it's out in front: The government of Trudeau says the legal smoking age should be 18, and not 21. (Sean Kilpatrick/The Canadian Press via AP, File)

  • In this April 20, 2016, file photo, a woman exhales after taking a hit from a bong during the annual 4/20 cannabis culture celebration at Sunset Beach in Vancouver, British Columbia. Legislation introduced in April 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. (Darryl Dyck/The Canadian Press via AP, File)