A while back, I remember reading something about DC9's having to be retired after they reach/get close to 100,000 cycles (1 cycle being one take-off and landing). I would assume the one being retired is close to that number. IIRC, the 100K number is where the FAA says the pressure bulkhead must be replaced or the plane must be retired. (apparently, replacing the pressure bulkhead is very time consuming and not worth the investment)