It is possible that a lot of peer pressure starts off the addiction to smoking and the will power to resist to peer pressure can easily crumble when one doesn't want to be left out of the group. Then there is the deliberate defiance. Let's say parents find out and the youngster deliberately continues, not liking the parental "interference" in his/her activities.
A side note: World No Tobacco Day is on 31st May 2019. It is to remind people of the bad effects of not only using tobacco products but also exposing others to poor health because of it. Passive smoking is equally dangerous. People are aware, as Natarajan said, but yet will continue with it. Many people just get hooked on to it, get addicted to such an extent that even when they make an effort to cut down, instead increase the consumption. Those who are committed to quitting the habit should have a strong support system, maybe a large support group of smokers who want to quit and caring friends to cajole, nag and coax them every day. If you know of someone with this habit, put the fear of God in them, send them pictures of oral cancer. Keep trying. Help them quit. For their own sake and especially for the sake of their family members.
When you make a commitment, you create hope. When you keep a commitment you create trust! ~ John C. Maxwell