I don't think how old an aircraft is matters that much.
I know there are quite a few KC-135s, 747 Classics, Lear 20s and 30s, DC-9s (older than any MD-8x or 9x), B-52's...all the way back to DC-3s. All these aircraft fly safely every day, amounting to thousands of trouble-free hours. What pilot would take off an aircraft of which he questions the airworthiness?
When the aircraft go through heavy checks, they come out ready to fly for a long time, stripped clean of corrosion, wiring intact, etc. In fact, I would rather take an old DC-8, overbuilt to a fault, through a thunderstorm before I would an A320 built last year to the exacting engineering specs of today's CAD/CAM.
We don't know what happened in Madrid yet, and must wait for the investigation to find out.
I'm so sorry for the families of the deceased, the people of Spanair, the crew and all others involved. This is a devastating event. As an airline pilot, I know the potential is always there for tragedy, but I comfort myself in the probabilities.
Over 100,000 airplanes flew today. One crashed.