It most definitely "dumbs you down" while stoned, but I must disagree with your blanket statement. Let's use me as an example: I test out in the upper 2% in I.Q. and can learn anything mechanical, theoretical, or just about anything. While not exactly proud of it, I started smoking dope at age eleven and stopped around 18. I did nothing in school, but get high, surf and work on fishing boats (a Spicolli if I ever met one).
I am certifiably sane, boringly so. My I.Q. has increased, according to tests, over the last 20+ years. I am not endorsing a THC clouded youth as the recommended way to grow up, but it is not as detrimental to sane people as the Gov is claiming. Remember, many of the kids who are getting stoned all of the time have crap parents who are not properly raising them, and who are often inflicting mental illness on these kids.