Myself I have two boys, one is 20 and one is 15yrs old. They both know that I did drugs when I was in high school that I just tried it. I did not have to tell them about my addication when I was in my 30's and early 40's they lived it. With both parent using in my house they only saw there dad always drugged up. So that was easy one. Now on the other had we were told by a doctor to tell our 15 year old that his dad was an acholic because he believed that the desease was in the genes. He has been told if he ever took a drink he has a better chance of becoming addicted to booze.
I am hoping that the choices they make are better ones then what I did. I would say just keep and open mind when it is time to talk to them. I would start talking about the age of 10. It is never to early to talk to your kids about drugs, sex and drinking. The more information they have the better they can maked the right choices for themselves.