If you damage your lungs by smoking, will it always stay damaged even if you quit?

Yes and No: Smoking does permanently damage your lungs. If you have smoked > 100 cigarettes in your lifetime, your risk of cancer goes up. However, if you quit smoking, emphysema worsens at a slower rate, and your risk of lung cancer goes down (though not to normal).

Controversial: The lining cells of the airways regenerate and function better after cessation of smoking. Approximately 10-15 years after you quit smoking your risk of lung cancer will reduce to those of a non-smoker. But some changes such as emphysema cannot usually be reversed. It is best to never start smoking!

Yes: I have seen patients with lung cancer more than 15 or 20 years after they have stopped smoking. If you stop smoking your lungs will not get worse from cigarettes. If you have not yet crossed the threshold, you may have prevented a cancer from developing.

Emphysema: Emphysema from smoking is permanent lung damage. It is not reversible.

Depends...: If you quit early enough in the damage, your lungs can heal to a certain degree but will never completely normalize. However, the closer to normal they can become, the less symptoms you will have and the more likely you are to avoid COPD or certain cancers, like lung cancer.