I am also a parent of grown children. I stopped smoking and drinking when I had kids, because I didn't want any chance of influencing them. Up until age 14 or so, I would give them no opportunity (lots of supervision) to drink or smoke anything.

During the ages 15-18, it was a don't ask, don't tell, don't get in trouble policy. They had opportunities to experiment, behind my back, and I didn't want to know.

As adults, they now can clearly make their own choices, and can talk to me openly if they have questions. They have shared some of the stuff that they "got away with", nothing earth shattering. Looking back, I think that they appreciated the limitations.