The research results on the effect of cannabis on developing brains is dubious at best. Some researchers claim to have found structural changes in the brain correlated with heavy teenaged cannabis use; others have not. None have found any long-term functional changes resulting from even very heavy cannabis use. The whole population data says there are none. Cannabis has been a popular drug among teens in Canada and the US for 50 years. If it caused functional brain damage, we could expect to see "pot brain" in all age ranges from 18 to 65, but we don't.

The data suggest that smoking marijuana provides a slight protective effect against lung cancer, and it is not shown to cause or aggravate other respiratory tract problems.

The evidence also is that cannabis, not combined with alcohol, does not impair the ability to drive.  

In short, this panel's advice is just a more sophisticated version of Reefer Madness.