Might it be that a a 20 or 25- year old get lung cancer?

Yes : Smoking, and some other carcinogens cause your own cells to be irritated, transform, then become malignant , and then grow a mass and then can metastasize in the process of cancer. Can happen at any age but more common with abuse and age! young people can also have other malignancies, lymphomas, metastases that come to lung!

Yes: Although it can happen, it is extremely unusual. There would have to be very early exposure to a stimulus for that to happen.

Yes: "from 2006-2010, the median age at death for cancer of the lung and bronchus was 72 years of age. Approximately 0.0% died under age 20; 0.1% between 20 and 34; 1.1% between 35 and 44; 7.8% between 45 and 54; 19.6% between 55 and 64; 30.5% between 65 and 74; 30.1% between 75 and 84; and 10.8% 85+ years of age" http://goo.Gl/5mixx.