Smoking costs an average of approximately ten years of life expectancy. Quitting before the age of forty eliminates about ninety percent of the excess mortality risk. Even quitting at a later age still yields several years gained, and the cardiovascular risk drops to the level of a never-smoker within ten years.
Quitting pays off at any age, but timing makes a big difference. People who stop before the age of forty avoid roughly ninety percent of the extra mortality risk that smoking causes. That is no small margin. Someone who quits at thirty gains back an average of about ten years; at fifty that is still six years, and even at sixty it is still three years. These are averages across large groups, so your personal gain depends on how much and how long you have smoked, but the direction is unambiguous.
What improves quickly after quitting is cardiovascular risk: within roughly ten years it approaches the level of someone who has never smoked. For lung cancer it works differently -- that elevated risk remains noticeably present for decades, even after quitting. That may sound discouraging, but it does not change the conclusion: quitting is the most effective step you can take right now.
The data come from some of the largest and longest-running studies on smoking ever conducted, including Doll's British doctors study spanning fifty years and an American analysis of more than one million women. The causal relationship is not in dispute.
Overview across multiple factors (2 research records, 4 sources). The strength of evidence differs by component -- read the answer for the nuance.