Notice how I made a point of only including Comair/ASA DLConnection carriers, which ARE 100% owned by Delta. Alaska owns Horizon, yet they do not count Horizon into their fleet age. Same for AA and AE. Same for CO and COex. Same for DL and Comair/ASA. True, it depends on the situation, but NW is the only one who counts its regional arm as part of its fleet age among this group.
Singapore Airlines, through its SALE subsidiary, owns aircraft that it leases. Yet SALE aircraft are not counted in SQ fleet age. From a professional standpoint, an airline should only count the aircraft it operates itself as part of its fleet age. Because NW does not operate the RJs, rather --Mesaba/Express I--, counting the ARJs and CRJs is not a best practice. Also, even though an airline may not outright *own* some of its aircraft, those aircraft are tallied into the equation because the airline operates them.
RJs simply are not the same as mainline jets, anyway. The planes have a less capital/maintenance cost, less amenities, less weight. I would assume there are dramatically different FAA and FAR21, etc. regulations as well between RJs and "real" jets.
Dont' be too offended. I know NW DC-9s have been refurbished and feature the 'space age' interior that businessmen love, but physically the planes are jalopies. I've seen the 9-30 cockpits... wow, what a mess. Now, the planes aren't bad, but from a maintenance and image-conscious perspective, it is not a best practice to have older planes. This should be fairly obvious. Notice how the only other airlines to operate planes as old as NW's are 3rd world carriers in Latin America... Avensa, Aeropostal, etc.
Don't be offended: NW does have a very good mx program that takes care of these babies well, but hopefully you get the point.