I flew some of the oldest A320's a couple of years ago in SE Asia. Numbers 300-311. They were 20 years old, but not a lot of hours. 28,000-30,000. A third of the hours they would have had at an efficiently run airline. They were complete junk, but part of that was the maintenance. Lots of airframe vibration above 300 knots, and the LG felt like it was held on by bailing wire.

I don't know how much of this was bad maintenance or bad airplanes. Last UAL one I flew was just over 4 years ago and they seemed fine although the number of MEL's was rapidly increasing.

I really don't know the maintenance side of the industry so I don't know how long they will last or how much it costs to keep them going.