The British Medical Journal studied smokers and their deteriorating lung function over a lifetime of inhaling.

By the time the study subjects first became symptomatically short of breath around age 58, they had lost 50% of their lung function. They were severely disabled by age 65 when their lung function was 75% gone, and by age 70 they were dead of COPD (chronic obstructive lung disease).

Those smokers, however, who quit at age 45 added 10 additional years on to the point at which they were first symptomatic, and did not experience severely disabling symptoms until age 75. And finally, those smokers who quit at last with the onset of severe disability at age 65 added 5 addtional years to their life.