A new study found this age is ideal for limiting negative impacts on long-term health, educational attainment, and mental wellness.

Determining the legal age to allow consumption of various vices is a matter that’s hardly ever agreed upon. Late last year, Donald Trump signed a bill raising the age of legal tobacco purchases in the United States from 18 to 21. The legal drinking age varies throughout the world; in countries like Austria and Portugal, 16-year-olds can legally consume alcohol, whereas in the United States and Sri Lanka, the age is 21.

States with legal adult-use marijuana set the age limit at 21, but that is a contested figure within the scientific community. Some researchers believe that number should be higher. As two physicians argued in the New York Times last year, states should set a minimum age of 25 and older to avoid the damage marijuana incurs on developing brains.

A new study out of Canada proposes a different conclusion. Published in the peer-review journal BMC Public Health, researchers found 19-years-old is optimal in limiting negative impacts on long-term health, educational attainment, and mental wellness. (The legal limit in most Canadian provinces, except Quebec, is 19.)

“While the medical community recommended an MLA [minimum legal age] of 21 or 25 based on neuroscientific evidence about adverse impacts of cannabis on cognitive development, this would lead to a large underground market for cannabis,” the study’s authors wrote. “On the contrary, policymakers have decided on a lower MLA such as 18 or 19 to curb the size of underground market, but this raises concerns about adverse outcomes for adolescents.

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“This study, however, found that later life outcomes associated with first using cannabis at age 19 are better than those associated with first using it at age 18 but not significantly different from those first using between 21 and 25.”

For researchers, determining minimum legal age is all about balance. Their findings reported different ages were optimal based on the quality of focus. For general health purposes, 18-years-old was the optimal age. To ensure the greatest education retention, 21 was ideal. And those worried about mental health should know the best age is 19, according to the study.

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“This finding helps to address concerns over potential adverse outcomes associated with setting a low age as MLA,” notes the study. “At the same time, our findings can also be used to highlight to youth and their caregivers, the long-term adverse impacts of starting cannabis before age 19.”