I believe in telling my children the truth. So when my eldest asked me about my own drug use - he was about 11 - I told him. I have smoked a bit of cannabis in my time, and enjoyed it, and I tried to give him a balanced view. I told him I had been 14 when I first smoked dope, that I had smoked frequently in my 20s, and that I might still, if someone offered me a spliff at a party. His first reaction was frankly startling: he freaked out completely and was incredibly distressed: his 'drugs education' at junior school had taught him that 'drugs kill' and 'you'll go to prison' - so he was sure I would die or be arrested imminently. He was basically too young to understand.
About a year later (though I didn't know this at the time) he was offered a spliff himself, and smoked some. By 13, he was smoking skunk whenever it was offered to him. By 15 he was smoking it almost-daily and had tried other things. Now he's 17 and he smokes skunk most days and takes M-CAT (and probably other stuff) when it's available.
I have talked to him about it countless times over the years. His response has been basically "Well, you started at about the same age" and "You did it, so you can't complain". He used what I told him as an 'excuse' in his own mind... Differences that feel quite crucial to me - particularly that I smoked someone's big brother's home-grown a couple of times between 14 and 16, while he smokes chemically modified 20-times stronger skunk daily - are irrelevant to him. :(
He doesn't see his drug use as problematic (and he hangs around with people who use far more) but I do... I see him 'switching his brain off' before he's properly learned to switch it on, and I worry... He is now doing nothing (not in education, not in work) and has stolen to get money for drugs (cannabis and M-CAT) and smashed things up and assaulted me when he's been coming down :( Basically, I think he started experimenting when he was too young to know his limits, and when the excitement was far more significant that any possible risks :(
So... It is the only lie I've ever wished I had told. If I had that moment again, I think I would duck the question, or even say "Me? Drugs? No, of course not!" I think I would be very tempted to keep the subject taboo until he was old enough to understand the risks as well as the glamour.