I would say if they do not need it medically then it should go just like alcohol, 21. If they need it then the dr. should be able to give you a good idea on how much they need. teenagers are gonna want to use with friends so i would say parents nee to be involved with their close friends and even meet the others parents. but yea if you need it and dont want pil form of drugs then marijuana is the best alternative if you dont need it for any medical reason wait till you are 21.