Lung cancer is also different. Smoker's lung cancer is a small cell lung cancer. As far as I remember, it is smokers who suffer from this type of cancer (more than 90%). An almost direct linear relationship between the number of cigarettes smoked per day and an increased risk of developing lung cancer has been clinically proven.
The chance of getting lung cancer in a smoker is AT LEAST twice that of a non-smoker. The amount and duration of smoking is superimposed on this.
Therefore, regarding your question, at the age of 20 you will get sick with cancer of a smoker with a very low probability, but continuing until 30-40 years and increasing the "dose" is quite likely.
It must be remembered that you will not get by with lung cancer. Cancers of the lung, oral cavity, pharynx, esophagus, bladder, renal pelvis, pancreas are considered to be causally related to smoking .
As a dentist, I can notice that in Russia it is simply not a trend yet, but in the entire civilized world, it is dentists who are engaged in detecting oral cancer at an early stage. We simply do not realize the relevance of this issue.
Lung cancer can get sick at 10 years old and non-smoker. Smoking is just one of the factors leading to cancer. One pack a day at age 20 will certainly increase the likelihood of cancer, but insignificantly if there are no other health problems, primarily the immune system.