Smoking killed Dh's mom, so he is pretty vocal about its effects. This can be hard for ds when people he cares about are smokers....

Also at 3 he was really interested whenever he saw anyone smoking. So here is my take on it all:
I tell him (particularly at 3--it seemed to be the age for that) that sometimes people do things that aren't wise. "Like what?" So I told him stories about things I have done that weren't wise (like not eating breakfast and getting really cranky), and other examples. People can be really great people and still make mistakes. I feel good about not hiding from him the fallibility of adults.
I also just told him the word "addicted," and said that when you are addicted to cigarettes it is really hard to quit smoking them, harder than almost anything. So it's important never to even start.
And I tell ds that not everyone gets sick or dies from smoking--that every smoker would probably be healthier if they quit, but that many smokers live a long time. It's a trick to try and balance the seriousness of the risk with not wanting to terrify him about his grandfather and uncle and friends. I tell him we can hope that the people we love will quit, and that they don't get sick, and if they are trying to quit we can give them extra hugs to help out.
My Df has since quit smoking (after many many tries), and DS was very happy. (Me too.) And he understood what was going on.
Also, be prepared for a possible bunch of play about cigarettes, if you haven't seen it yet. Ds was obsessed for a while. He talked about smoking, pretended to smoke, claimed he would smoke when he grew up, etc etc. His friend the same age did it too. Not so much any more--just letting you know so you can take it in stride.
