"When that random stranger chastised me for smoking, it totally made me want to quit," said no smoker ever.
You know what random comments from strangers and friends alike do? They make an addict want to light up. Because an addict knows it's bad for her, she knows it may one day kill her, she knows it's expensive, and those little reminders from "honest" people just make them stressed and more likely to have a craving.
And I think by age nine, most kids are capable of understanding that you're not supposed to comment on other people's behavior and appearance.
ETA: I speak from experience here. I was only able to stop smoking by convincing myself that I wasn't really quitting; I was just waiting an extra 5 (10, 20, 60, and so on) minutes before indulging, and if I really wanted one, I could have one. Telling myself I had to quit so I didn't get lung cancer was a surefire way to fail. Years later, I still allow myself to light up if the craving is extremely strong. This only happens about once a year, so I'm pretty sure it's not killing me.