If things that cause some harm should be criminalized, so that age of consent people go to jail if they participate in using them. 
Then why are we not throwing in jail:
People who use alcohol
Smokers
18 year old children who choose to joint the military, (to get blown to bits in Afghanistan)
People who buy firearms.
Adults  should be able to make personal choices regarding themselves as long as harm to others is managed. Arguably alcohol and joining the military present much more risk to others than does cannabis use.

Labels: moderation: safe