The average life expectancy for men is 76 and for women 80. Smokers die about 10 years earlier than non-smokers. But since we all have to die of something, it might as well be something you enjoy -right?
Age 45 to 50 seems to be the tipping point. What your doctor should say is, "How do you want to live the last 20 years of your life? If you stop now, you will probably stay healthy and be able to continue doing the activities you enjoy most. However, if you keep smoking, you will probably end up disabled from heart disease, cancer, stroke, or emphysema."
Here is a graph that shows the progression of lung function decline of smokers, non-smokers and quitters leading to disability and ultimately death.