This is not the scientific consensus.

In fact teen drinkers who also smoke cannabis may suffer less brain damage than teen drinkers who do not smoke cannabis:

"Results confirm previous studies linking adolescent heavy drinking to reduced verbal learning and memory performance. However, this relationship is not seen in adolescents with similar levels of alcohol involvement who also are heavy users of marijuana."
[Mahmood et al. Learning and memory performances in adolescent users of alcohol and marijuana: interactive effects. J Stud Alcohol Drugs. 2010.]

"Teens reporting binge drinking behaviors alone had significantly lower FA than controls in all eight clusters, whereas teens reporting both binge drinking and marijuana use had lower FA than controls in only three of the eight clusters."
[Jacobus et al. White Matter Integrity in Adolescents with Histories of Marijuana Use and Binge Drinking. Neurotoxicology and Teratology. 2009.]

Even the U.S. Government has a patent on the cannabinoids found in cannabis for protecting the brain:

"This new found property makes cannabinoids useful in the treatment and prophylaxis of wide variety of oxidation associated diseases, such as ischemic, age-related, inflammatory and autoimmune diseases. The cannabinoids are found to have particular application as neuroprotectants, for example in limiting neurological damage following ischemic insults, such as stroke and trauma, or in the treatment of neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease and HIV dementia."
[Patent 6630507 - Cannabinoids as antioxidants and neuroprotectants. Filing date: Apr 21, 1999.]

In this study, researchers observed teens brains over a 1.5 year period. They found that teens who used alcohol during this period had reduced white matter integrity. These changes were not observed in cannabis users:

"More alcohol use during the interscan interval predicted higher mean diffusivity (i.e., worsened integrity) in right and left superior longitudinal fasciculi, above and beyond baseline values in these bundles. Marijuana use during the interscan interval did not predict change over time."
[Bava et al. Longitudinal changes in white matter integrity among adolescent substance users. Alcohol Clin Exp Res. 2013.]

That said, there are concerns about regular, heavy teen use and adverse effect on the brain. As it stands over 80% of High School seniors report cannabis easy to get. This has remained unchanged for the last 30 years. Putting it behind the counter where merchants will actually check ID can actually reduce the regular, unfettered access that they have now. Regardless, criminalizing adults for something that children should not have is not rational or consistent with our legal system.