Well, it's like this, in my mind.
There is no age too old for an airplane. Usually.
For example, there are DC-3's that are 60 years old still making nostalgia flights in civilized countries, and scheduled flights in countries... less civilized.
However, we all know that metal DOES age and crack- witness the Aloha 737 incident in Hawaii. Also, wiring ages- witness the Swissair MD
-11 incident over Nova Scotia. That might not be the best example, though, as that MD
-11 was not yet ten years old at the time.
SO- if PROPERLY MAINTAINED, there is no age too old for an aircraft. But I don't think the families of the dead flight attendant from Hawaii could be convinced of that.
Midwest is flying the second DC-9 ever produced- it rolled down the lane at Long Beach in 1966. It's safe. It's not on fire.
Engine fires happen, unfortunately. And, as you pointed out, have little to do with the age of the airframe.
So I guess the answer is one you have to form yourself.