This fatigued aircraft stat is curious. I still can't figure out what it means, but that's ok.
Its really quite simple. Just look at it as a 'tired' airframe. Stress cracks, popped rivits, frayed cables, worn tires, battle damage, etc. Landings on carrier decks are rough on planes, even today, so over time the airframe weakens. Happens to all aircraft, moreso for carrier planes, and AE being the game it is takes this into account.
Edit:Ultimately it'll mean more battle and op losses unless its reduced.
Think of your planes as race cars. Built for performance, not longevity.