The jury is still out on the negative effects, but it would be whistling in the dark to assume that there are no ill effects.
https://www.cbc.ca/news/health/cannabis-canada-giant-public-health-experiment-1.4865606
I don't believe there will be much - if any - difference in the rate of usage among minors after legalization, because essentially, nothing has changed for them. I am concerned however, that the increasingly relaxed attitudes towards cannabis will lead to more incidents of small kids consuming edibles and ending up at an Emergency Dept. and dogs getting into stashes, since they will eat anything. I've been hearing more stories about this recently - even before legalization. Now, people will be even more relaxed about storage, leading to more such incidents.
"Brain damage" might not be an accurate description, but there is strong evidence pointing to brain "re-wiring" amongst young, frequent users.
As a moderate drinker, I won't be hypocritical about the whole issue. The devastation caused by alcohol far, far exceeds that caused by cannabis. But, don't pretend that cannabis is harmless.
To me, alcoholic drinks are beverages. Inebriation is simply a by-product. I very rarely have more than one or two drinks at a time now, to avoid that effect. In my younger days, drinking to excess was not exactly rare, but rare enough that I never developed a dependency.
For me, the big difference between alcohol and cannabis is that alcohol is a beverage with a plethora of different forms and flavours to enjoy, while recreational cannabis usage has just one purpose - to get high (to varying degrees). I have no interest in having my mind altered, as I'm quite content with reality.