Before they were retired USAir had some -9s with reportedly over 90,000hrs on them. And NW (ne Delta) has some -9s that are as old as the USAir birds and they are still flying albeit with new paint.

We were in the hangar when an -80 was going through a major check and it was down to the bare bones. Everyone wondered how in the hell they were going to get it back together. A week later it was plowing the skies.