The percentage of smokers who actually get any kind of cancer is quite low, about 20%, and not all of that is lung cancer. It really depends on the person and their overall genetic makeup, but on average, smokers will get lung cancer in their late 40's and up with women getting it younger and men getting it older.
And even quitters aren't exempt completely. There have been cases where men and women quit in their 20's and 30's who have still gotten lung cancer.
And yes, I know I said it was low, but still...don't take that risk. I smoked for years and years and have finally stayed quit for 1 year and 6 months and I know I won't be out of the woods completely.