We don't.

http://www.cps.ca/en/documents/position/cannabis-children-and-youth

"Youth should not use cannabis recreationally because its many potentially harmful effects are serious. These effects are present in the entire population; however, the developing brain is especially sensitive to the negative consequences of cannabis use. Canadian youth are at significant risk for developing CUD and, possibly, for doubling their risk of having a psychotic illness. Driving under the influence of cannabis increases the risk for motor vehicle accidents. Where cannabis has been legalized in the United States, children are requiring emergent medical care at greater rates due to unintentional ingestion...Based on the physical and mental health risks, and with many legal, financial and public safety issues at stake, policy makers – with support from physicians and the public at large – must continue to limit access to cannabis."

We should be listening to Canadian physicians, however.