When should I quit smoking?
The answer is: right now. Improvements in pulmonary function can be achieved at any age. The following graph illustrates that quitting at the age of 45 can prevent disability in the average person; even quitting at 65 can delay death by an average of 25 years.
Graph by Boehringer Ingelheim Pharmaceuticals, Inc. which was adapted from Fletcher CM, Peto R: The natural history of chronic airflow obstruction. Brit Med J 1977; 1:1645-1648.