George Wallace said:So we should totally ignore the fact that scientific research has found that the human brain is still in a state of development into the early 20's, just because we have decided that a person legally becomes an "adult" at an earlier age?
So - I'm not really sure what your argument is here. We have drawn arbitrary lines all over the place. The legal age for alcohol is arbitrary. After all, it's carcinogenic, and there is no safe level of it. It causes many deaths. The same is true of tobacco. I would argue (as you seemed to argue otherwise in an earlier post) it matters very much the precedent created with those drugs. I would also argue that marijuana is far more socially accepted than you understand.