My 1994 Trek 5500 carbon fiber is 28 this year. It has lots of dings in the paint due to locking it up in the over crowded employee bike enclosure and the lovely Campy Chorus groupset is now pretty sloppy. As a young engineer I worked at Thiokol (rocket motors) and my frame and fork does not have a fatigue limit as said but will degrade from fatigue, differently from metals with a limit which typically surface crack. Fatigue in carbon lay-up can break fibers, cause resin bond to fiber failure or cause fabric layers to separate. Fortunately these conditions can usually be heard if one taps on the fork or frame and the sound is dull, more thud rather than the sharp sound one would expect.