Besides, I think there are two important jobs you have to do as a parent:
you parent by actual prevention
and you parent by what you stand for
If my 13yo came to me and said I want to start smoking, I could of course say to myself, oh there is no way I can stop this, he can always get hold of money and get some from an older mate, so I might as well buy him some cigarettes myself.
Or I could say to him: look, I think this is a bad idea for x and y reason, I know I cannot lock you up until you are grown-up to stop you, but I don't want you ever to think afterwards that I sanctioned this. If you do it anyway behind my back, then it is your decision and I want you to know that.
This shows him a) that I really am serious about wanting to shield him from harm b) that I really am serious about this being harmful c) that I have respect for my own integrity and that noone (not even my beloved child) has the right to force me to do something I think is wrong.
I hope I would be able to do this in such a way as to make him feel I was still listening and still caring about his pov.