Huh?  I can't see how you could interpret in what I said that quitting is not essential ASAP for any smoker.  Isn't that what I said??  The overall benefits are immediate.  But..it is true that the carcinogenic risk does not reduce to near nonsmoker rates (here I mean people who never smoked) for 15-20 years after.   So yes, if you smoked heavily up until the time you were 30, then quit...your health will/did improve immediately after quitting, but your lifetime lung cancer risk will not approach a non-smokers until you remain quit about age 50. 

Both Jennings and George Harrison resumed smoking, but there are many people who quit and 10 years later or so, do unfortunately get lung cancer. 

 Does this mean I'm doomed twenty years from now? No. Not at all.  The risk of getting lung cancer after smoking goes down immediately as soon as you quit.  It then continues to diminish with each passing year, but it does not approach a non-smoker's risk (which is not zero BTW), until 15-20 years post quit.   Not all lung cancer is smoking related.  In fact, about 40% of all lung cancer is not directly related to smoking.  Christopher Reeve's wife was just diagnosed with lung cancer, and I suspect hers was not smoking related although I don't know for sure if she was ever a smoker.  There is an increase over recent years of small cell lung Cancer which is not necessarily secondary to smoking.  Fortunately, there is very effective chemotherapy for that type of cancer, so that patients can live for several  years with great quality life after diagnosis. 

Even smoking related lung cancer can be cured if detected early.  I've seen several patients who've had single lung tumors surgically removed and who remain cured 10-20 years after surgery.