I quit smoking at 36 years old after 15 pack years. what's my risk of lung cancer?

What is done is done: However, if you can go the next ten years without smoking, your risk will come back down to that of a non-smoker.

Difficult to predict: It is good that you are quitting smoking. It takes a while for the body to recover from the damage of tobacco and your risk will be lower due to quitting than it would be otherwise. I wish you the best in your effort to quit use of tobacco.