I was a baby, can't remember how young. One of my best friends was diagnosed at 30. A lot of people aren't diagnosed until later is because they don't show a lot of the classic symptoms, or don't start showing them until later in life.
The only reason I was tested was because I tasted salty - the doctors told my parents it wasn't necessary because "there was no way I had CF", but my mom wanted piece of mind. I don't know if my parents or the doctors were more shocked when the results came in.