Could you develop cystic fibrosis as a teenager?

No: A small segment of patients affected by cystic fibrosis does not show typical symptoms until teenage and apparently has very mild disease. This depends upon the mutation involving cftr gene (homozygous vs heretozygous so on).

Genetic disease: CF is a genetic disease so you have it at birth, but it is possible for the diagnosis to be missed until later in life.

Yes diagnosed then: CF is a genetic disorder which can be detected at birth. It is possible to present later in life with symptoms suggestive of cf. A CF sweat test should be performed if your doctor suspects cf.