It's almost always, not until later in life, that some smokers realize the damage that smoking cigarettes has taken on their health, and decide to quit. But by then, the irreversible damage has already been done.
It's not that smokers aren't aware of the risks, they just think it won't happen to them. Just ask lead singer and founding member of The Hollies, Allan Clarke. If only he wouldn't have passed on his own needs early in his career, and quit smoking back then.