Anti-cannabis campaigners say the minimum age of 20 to use and purchase weed wouldn't stop young people from feeling the harm of legalisation.
Marijuana can halt motivation for young people, leading to them dropping out of school and stunting their development, and it can cause mental health issues like psychosis.
But those issues are already happening in New Zealand, and international examples show Canada's youth usage rates dropped after legalisation.
Diana Ranger has seen the worst of cannabis use in young people.
Her nephew was forced to take the drug by a gang when he was 10, and she said he suffered from psychosis as a result.
"He was going in and out sanity and insanity, and he bit me right through the arm," Ranger said. "I didn't know a human could break the skin and leave actual bite marks.
"Then he'd recover, and he'd lose his mind again. He'd shove his fist right through windows.
"It took all night with the help of one of my sons to recover."
Ranger said that happened 40 years ago, and the man is still feeling the effects today.
These sorts of stories cause serious concern for Aaron Ironside, who leads the Say Nope to Dope campaign opposing legalisation.
He does not think an age limit on purchase will stop kids from trying it, as is the case with current alcohol laws.
Ironside said when the brain is developing, you do not want to be using substances that could impact on that development.
"The risk of psychosis, the risk of cannabis use disorder, is four times higher for adolescent use, but also it's in the flow-on effects of poor educational outcomes which in turn lead to poor employment outcomes."
There are no arguments from doctors around the harm cannabis can cause, although there is debate over the severity.
Professor Joe Boden from the University of Otago was on the expert panel formed by the Prime Minister's Chief Science Advisor, looking into cannabis.
He said psychosis is a concern, and is already happening to young children in New Zealand under an unregulated market.
He said the concerns around how it impacts schooling is also more complicated, and not necessarily health related.
"You have to keep in mind of course that some of that is due to school exclusion because cannabis is one of the few things that you can be automatically excluded from secondary school in New Zealand for," Boden said.
"I'd give the example of one of my daughter's friends who was kicked out of two different high schools in Christchurch, for cannabis."
Ironside said he is also concerned about the second-hand impact of smoking cannabis.
"We're very concerned to read this study that suggested up to 50 percent of the children, in this particular study, where they were inhaling second-hand cannabis smoke, had THC in their urine," Ironside said.
"We've heard from many teachers who can give first hand accounts of the children who come from homes where cannabis is being used, that the children are clearly affected, clearly unable to engage in a days schooling in the same way that their classmates are able to."
That study included 83 children, and was conducted in Colorado.
Ironside also pointed to a study which showed children exposed to second-hand cannabis smoke may be 80 percent more likely to suffer adverse outcomes.
That study, conducted in California, included 29 children who had been exposed to cannabis.
Boden said there is a lack of quality research on the impacts of cannabis, largely because it has been outlawed in most places.
While he said there can be harms to young people, he said that is happening now and there is no evidence it would get worse under legalisation.
In fact, he said it could improve.
"In Canada, in 2018 which had about two months and 13 days of legal cannabis, 19 percent of people in that age range reported using cannabis in the past year.
"In 2019, which is an entire year of legal cannabis, 10 percent of that group reported using cannabis, basically cutting the rate of youth use in half."
Boden said that study had more than a thousand responses.