In regards to design life of the DC-9 or any aircraft for that matter, its how it is maintained and operated. With all the new non-destructive testing that can be done now days, there is virtually nothing that could not or would not be detected and repaired.
With the Aloha accident, that airplane had a large number of cycles and most airlines and manufacturers didnt know what cycle fatigue would do to an airplane. Well, obviously we do now and inspection procedures have been put in place to prevent this from occuring again.
Northwest, I'm sure, has spent lots on there enginering department developing an inspection plan to ensure there DC-9's are maintained to the highest standards. I wouldn't hesitate one bit to climb aboard one of there oldest and go for a ride.
I have faith in the system and I have worked aircraft overhaul and especially on the DC-9, enough to know, its a sound machine. But I still prefer the old three-holer Boeing any day!