What's the youngest a person or child could develop lung cancer from second hand smoking?

Hard to say.: Lung cancer usually, if not always develops in people over 40 with the peak incidence between 55 and 65 years. Cases under 40 are rare regardless of cause. However the international agency for research on cancer, the U.S. Surgeon general, and many other agencies have classified secondhand smoke as a carcinogen, and many U.S. Studies showed ad increased relative risk for cancer in those exposed.

Very rare: In children lung cancer is basically non-existent. Mean age is 71 year old. Age 55-79, with a 30+ pack history have the greatest risk for lung cancer. Direct smoking is the major risk factor (85-90% ofcases). Sencond hand smoking is a relative risk factor , the real risk not clearly defined. I still recomend avoiding 2nd hand smoking.