I think it depends on the kid. I talked to my 12 year old nephew about it because he knows his mom smokes but she never talked to him about it, and he knows I smoke as well. He's really emotionally mature and seemed to understand everything I told him. I know its different when its your own kid; i have the benefit of just being the cool aunt. Also, only you can judge if your kid is mature enough to understand that you use it responsibly and it rarely becomes a problem like other drugs. Either way, I definitely think you should start educating him on marijuana right away. He's probably already been through a dare program or something similiar in school and therefore has already been brainwashed to believe that pot is a bad drug. You should start immediately to reverse the brainwashing. If you educate him and let him know your personal views on marijuana without letting him know that you personally partake in it, you will also avoid looking like a hypocrite when he does eventually find out you smoke.
When I was like 9 I found out my mom smoked and I hated it. It's different for many situations though.