Marijuana is harmful to the developing brain. The brain goes on developing until the early twenties. Studies show that teenagers who smoke marijuana have a much higher rate of schizophrenia and learning disabilities. They also have a much lower rate of achievement at school and ultimate success (measured by college graduation and future income).
I am all for making pot legal for adults. But, I think you have to use a similar age limit to alcohol...21 years. Any earlier than that and you may harm the developing brain of someone who is not quite able to grasp the long term consequences of their actions (don't kid yourself teenies). Any adult who wants to sit home and smoke a joint (or even crack in my opinion) should be able to do so... just as long as they understand the consequences of doing stupid **** while impaired.