What is the risk of lung cancer of a person who has quit smoking 16 years ago compared to the normal person who has never smoked before?

It would depend on how much the person smoked. The risk of cancer does decrease with passage of time but a quantitative estimate is not feasible. You can reduce your risk of cancer by adopting healthy habits: Eat a mostly vegetarian diet, avoid alcohol, tobacco and street drugs, avoid excessive exposure to sun, get HPV vaccine, exercise 30 minutes each day, drink enough water to stay hydrated.