If you are 58 years old and you smoked all of your life, at what age would you get cancer?

No set answer. However, this is a common age to develop it...And also a perfect age to stop since the risk will continue as long as the person smokes. The reverse of this is that the risk of developing lung cancer goes down significantly after quitting for 5 years and is the same as a non-smoker by 10 years...So now is the time to stop!
From now onward. Your risk from your smoking will exist from now till the rest of your life. It's best you stop smoking. However you may still get cancer even if you stop. There are people who smoke and never get cancer. Always pay attention to symptoms and see your family doctor for all the appropriate screening.