When my eldest was a senior in HS at 17 she found our pot and, since we'd had many conversations with her about the various intoxicants, she came right to us and said WTF? Turned out she had no clue we were regular smokers and she had been smoking for a couple of years on her own. I had told her that pot was better than alcohol, but she should wait until she was older to use them because of the potential for cognitive development issues. I didn't start until I was in my twenties but my wife started in high school and if that didn't demonstrate the case, nothing would.
Anyway, she's a productive member of society now at 21 living on her own and paying all her own bills. My only rules were she couldn't smoke with us until she turned 18. My two other girls, 15 and 13, already know we smoke and neither has shown the slightest interest. In fact, I call my 15 year old Grandma Prude because she's somewhat disapproving of our lifestyle. She's got a very strong moral sense and always tries to do the right thing, but she's no narc fortunately. We've had lots of conversations on the subject and as she grows up, she's getting more understanding of the difference between illegal and immoral.
It's tough to be a parent, but I've found the key is open communication and understanding. It also helps to get lucky and have good, self motivated kids with strong personal morals.