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.
The age of an aircraft matter a lot.
Older aircraft crash more and suffer from more issues due to age and design features.