What are the possibilities for a 20 year old to get lung cancer ? i mean is it possible at such a young age ?

Not much: Cancer of lung at the age of 20 is very rare. If you are afraid of getting lung cancer don't smoke , don't get exposed to toxic chemicals including second hand smoke , have clean habits, then you will reduce the incidence in later years of your life. Who knows 30 yrs from now when you are 50 there may be even cure good luck in your life.

Low: "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.