I am not sure you are making a good case here. Guns are sold to people 18 and older. However, there are numerous cases of folks owning guns getting blown away by a toddler or kid. Or kids killing themselves with them.

Pot is not really entirely harmless, I am a bit confused as to why that even matters. If you smoke a bunch of weed and then drive a car it is pretty dangerous. And there are some studies that indicate some detriments.

And not sure one counter example proves your point very well. If you look at the states with the higher rates they more times than not have the more lax gun laws. You can refuse to believe that all you want but it is the reality.

I would also say that it may not even be the ease of acquiring guns in those areas, but I would wager that folks there have more access to guns.