Minimum legal age varies for all kinds of grownup things. Alcohol, tobacco, and cannabis can be 18, 19 or 21, depending on state, province or territory.

Learning to drive is between 14 and 16, with agricultural states tending to allow driving at a younger age. The age of consent is 16 in most US states and throughout Canada. At 18, you can vote and even run for political office in both countries.

When it comes to legal cannabis, the random collection of ages across North America is curious. Every US state that allows recreational cannabis sales requires customers to be at least 21 years old. In Canada the minimum age is 19, except in Alberta (where it’s 18) and Québec (which started at 18 but raised it to 21 earlier this year).

In most jurisdictions, medical marijuana is legal for people age 18 and older, with a doctor’s recommendation.

What difference does it really make if someone is 18, 19 or 21? A research team at Memorial University in Newfoundland, Canada, recently investigated the question.

Instead of looking at the immediate health and safety of young adults, they assessed later life outcomes—namely educational attainment, lifetime cigarette smoking habits, and general physical and mental health.

In their study, the Memorial University team concluded that the ideal minimum legal age for cannabis was 19.

Looking at long-term effects

While age 19 tracks with minimum legal age in most Canadian provinces and territories, the study’s lead author, Hai Van Nguyen, says his team’s study wasn’t about questioning or confirming current regulations. It was about measuring the impact of a long-prohibited substance that has not been well evaluated, and the long-term effects of its use in early adulthood.

“We don’t take satisfaction from whether our findings support or contradict the current minimum legal age in most provinces,” he said. “We believe our findings can help inform the debate on minimum legal age in Canada, as well as inform policymakers in other jurisdictions that are looking to legalize non-medical cannabis such as Mexico, New Zealand, Russia, Luxembourg and several US states.”

Medical groups like 21

Who else influences policymakers? Health organizations, for one. While the American Academy of Pediatrics declined to comment on the study, a spokesperson did point Leafly to its policy statement on cannabis:

“Given the data supporting the negative health and brain development effects of marijuana in children and adolescents, ages 0 through 21 years, the AAP is opposed to marijuana use in this population.”

In 2019, US Surgeon General Jerome Adams released his own advisory on cannabis use. Adams focused on concerns about the effect of cannabis use on the developing brain, and he concluded with a “nationwide call to action” on cannabis safety for young people. Adams didn’t get into specifics on age, but expressed concern about cannabis use while the brain continues to develop into a person’s mid-20s.

The science is still emerging

That being said, the science around early cannabis use and brain development is still evolving.

Health experts cite THC exposure in adolescents causes changes to the brain’s folding patterns, decreased neural connectivity, thinning of the cortex and lower white matter, among other symptoms. However, one recent study suggests any changes to brain structure caused by cannabis use in adolescence cleared up by the time subjects were in their 30s.

Another ongoing study in the Saguenay region of Quebec took MRI scans of over 1,000 adolescent brains in 2002, and the same subjects are currently being re-evaluated as adults—results pending.

Why not make it 25?

If the serious nature of brain health is such a risk, why not just make cannabis illegal until a person’s mid-20s?

In an ideal world, sure—and in this ideal world underage kids never go looking for cannabis from illicit sources, either. In the real world, though, policymakers have to weigh human nature’s penchant for the forbidden with appropriate rules and consequences. In an ideal world, alcohol would also be outlawed for health reasons, but we all know how Prohibition worked out.

Prior to the Oct. 2018 opening of legal cannabis sales in Canada, a government task force took a hard look at the best-legal-age question. That group found that the higher the minimum legal age, the more likely adolescents will seek out unregulated sources, risking both consumption of potentially more dangerous products and also incarceration.

Canada took its cue from alcohol

As health groups pushed for minimum legal ages of 21 and even 25, Canada’s Cannabis Act ultimately left it up to individual provinces and territories to set a minimum age. Most ended up following the rule already in place for alcohol: 19.

Douglas Berman, law professor at Ohio State University and executive director of the college’s Drug Enforcement and Policy Center, underscores the individual and community harm that comes with arrests at a young age, “and the collateral consequences of possibly having a conviction and everything that flows from that.”

While sensitive to the science suggesting THC may have long-term negative effects on the developing brain, Berman points out that the federal minimum legal drinking age was largely based on driving and road safety, not brain health.

On that note: The US federally increased the minimum legal drinking age to age 21 in 1984. Before then, the minimum age varied from state to state, anywhere between ages 18 and 21. Prior to 1970 in Canada, the minimum legal drinking age was 21, but was then lowered. The countries essentially swapped.

A holistic approach to the minimum age

Nguyen’s study, which landed at age 19 overall for cannabis, revealed different outcomes for different ages:

• A minimum legal age (MLA) of 18 was associated with good general physical health.
• An MLA of 19 correlated with no cigarette smoking and good mental health.
• Under an MLA of 21, results showed higher educational attainment.

Do those small differences in age matter? Yes and no, says the study’s lead author.

“I think one way to look at the three-year difference between MLAs for general health (18) and for educational attainment (21) is that it suggests setting an MLA involves a tradeoff and requires a holistic view to balance different goals,” says Nguyen.

He points out that the pattern of higher education in young adults who first consume cannabis after age 21 is consistent with the medical community’s concerns about cognitive impairment on the developing brain “which in turn might adversely affect education attainment later on.”

Different factors go into ‘health’

Years of post-secondary education isn’t the only measure of overall health, however. And that’s not a goal, or an attainable path, for everyone. That’s why Nguyen’s study placed equal weight on mental health, general health, and cigarette smoking prevalence in addition to education, which all contribute to lifelong individual health.

“If we are willing to give equal weight to all these four outcomes,” he says, “then the overall optimal MLA should be 19, which is the average of these four individual MLAs for these outcomes.”

Maybe a graduated system: Low THC at age 18

From a legal standpoint, Ohio State University law professor Berman says he wonders if a solution to cannabis minimum legal age could be found in a graduated system, mimicking an outdated Ohio state law whereby 21 was the minimum age for wine and liquor, but lower-alcohol beer was accessible to 18-year-olds.

“I think it’s interesting to imagine THC potency limits paired to different age groups, and if that could be a way to unfold policy,” Berman says. “So, younger people would have legal and safe access to low-THC products that would still get them where they want to get to. And then, OK, you’ve hit 25 and your brain has stopped developing, we’ll let you have access to some higher levels.”

Berman recognizes this idea isn’t perfect, and could be a challenge for the commercial industry. But he says at least this theory incorporates some of the emerging brain science, while recognizing that young people are accessing cannabis anyway—legal or not.

Watching Québec closely

Nguyen says his study will continue as more data becomes available. He’s next focusing on Québec’s recent cannabis minimum legal age increase from 18 to 21. “It’s important to study if this higher MLA will translate into better outcomes for youth as the Québec government expected,” he says.

Berman is also watching the changing legal landscape, and says the different age threshold between Canada and the US allows both countries to learn from each other’s experiences and move forward based on evidence.

“I’m sure people will be debating whether this harm or that consequence is a result of this policy or that policy,” he says. “But being open-minded and nimble about how we approach legalization is where, in my own thinking, best practices come out.”