Your kids are way old enough for you to be discussing this all logically with them. I raised my D19 on logic, and so far, she has never steered wrong - it simply makes no sense to. My H asked her once why she doesn't get in trouble all the time like her friends (who were always getting grounded for this or that), and she just said 'Why should I? It's stupid to do something you know you'll get punished for. Nothing is that important.'
I 100% believe she is like that because since she was little, I talked to her about everything from a logic standpoint.
If you date that boy who has dumped 3 girls in one month, what do you think is going to happen?
He'll dump me, too?
Probably. Is it that important to say you went out with him, when you're pretty sure you'll break up in a week?
No, not really.
Or...
So it looks like Jennifer has now progressed from pot to pills. And now she's failing math and hanging out with Bill and the other druggies, just like we discussed happens when you start on drugs - the progression. (she tells us all this stuff) What are you going to do about it?
Well, I told her I can't be friends any more if she's going to choose drugs, but that I wish she'd just try to do without them, cos I want to still be friends. She just said too bad.
Does that make you feel bad?
It hurts my feelings, but I'd much rather find a friend who thinks friendships come before crazy stuff like that.
Thing is, kids are sponges. If you talk to them as equals (at your kids' ages), let them know your fears, your experiences, your expectations...they want to hear it all. They want to learn, and their parents are the two people they trust the most to know the 'truth'. So just keep providing it. Basically just start talking out loud, all the time, and invite them to chime in.
One thing that worked the absolute best for me was to take regular walks with D19, when I'd get home for work. Sometimes 2 or 3 times a week. It was a non-threatening way (you're not facing each other, thus no confrontation) to get off your chest your stuff, and listen to what your parent has to say, without it having to be a 'lesson.' And nothing is more uncool than a lesson!

We had a dog, so we had an excuse to walk the dog, but after awhile, she just wanted to walk. And boy would she get mad if H called me in the middle of HER time, lol!