I wouldn't recommend any alterations in brain-chemistry at an age when the brain is still developing. Cannabis certainly isn't guaranteed to be detrimental, I have a 20 year old friend who's been smoking pot every day since she was 13, and she has one more semester to go before getting her degree in business administration. My other hardcore stoner friend is in his third year of sciences, getting all A's and B's, majoring in physics, chemistry, evolutionary and nuclear biology, as well as advanced calculus.

On the other hand, through the teen years synapses are forming and grouping, the brain is pruning and making connections more efficient, the hypocamupus is still developing in memory and emotional control (THC has been shown to stimulate cell-growth in the hipocamupus however). The point is, consuming a substance which will (while tolerance is low enough to still induce pleasure) stimulate seratonin and dopamine production at heightened levels, will be bound to carry a risk of descencitizing those receptors (or "burning them out") and cause issues with depression, emotional stablity, and occasionally psychological instability. Not to mention such a stress-relieving and often escapist substance used in the teen years will deprive the brain of the necessary time needed to develope skills in dealing with stress and responsbilities independantly.

I say legalize cannabis, but it's not for kids.