When I need clear a special term or a fixed expression, I use both google and special native language sites. For example CDC or ALA don't use the term "child smoker". They use expression "child who uses tobacco" or "smokers among children". But I'm not native speaker and I can be wrong.

Statistics on adult, i.e. people old-enough to smoke are easier to come by, which is why you see it so often. If you're lucky, you'll only hear about teen smokers. When you hear about child smokers...we just failed as a society.