One point I haven't noticed in this thread was those who are "perma-fried". A former co-worker, guessing 28 yrs old) is in such a state. He was always slow witted it seemed, never seemed alert, had the "Beavis and Butthead"(sp?) laugh down pat.
I have found these type of people are the ones who get stoned on a daily basis. It's all about being responsible, I have friends' parents who smoke MJ like one drinks beer. You don't drink before work, you don't drink after work. You drink in moderation.