As with everything, it should be up for the parents/children to decide when they are mature enough to experiment with mind altering substances. I firmly believe that the parents should be able to provide a safe setting for their children to do so (if they feel it is appropriate) without risking going to jail.
How do you decide when they are mature enough?
Edit: To be more specific. How do I decide if a 7 year old is mature enough if they seem to want to do a certain drug?