There are mild cases of cystic fibrosis. A fairly good determinant of severity is the type of gene mutation that a person has. You must have two defective CFTR genes in order to have cystic fibrosis. The specific genes that are inherited have some bearing on the severity of the condition. For instance, the most common mutation in white people of European decent is called delta F508, and it is more likely to have a more severe course.
The life expectancy for the average person with cystic fibrosis is between 30 to 40 years old.