After you quit smoking your lungs can begin to heal slowly and it depends on how much you've smoked before then.
If you've smoked way past 50 years old then your lungs will be damaged more severely than if you stopped smoking by age 30 or 40.
Still smoking at any age is a good thing for your health and your lungs will start to heal and you'll have a much easier time of breathing.
You decrease your chances of getting lung diseases and COPD and even lung cancer the sooner you quit and it's always worth it to quit smoking even when you've been smoking for 30 to 40 years or more.