Given these were brand spanking new in the early 90's and the USAF (and ourselves) are happily flying '135's that are a good 25-30yrs older, and that they've hardly been flogged to death then I'd describe them as anything but worn out!
Although built at the end of the 80s, the mission system was designed at the end of the 1970s, and the current E-3D still uses the same computer etc. Bearing in mind that this is contemporary with the Spectrum 16 and the BBC B then you can see why it is getting difficult to find spares. Buying COTS instead of original designed equipment leads to a huge flight testing effort - the mission system is not only for the aircraft but has to give an appropriate service to other users and has safety implications for them as well. Despite its age, the Sentry is a marvellous piece of kit but unfortunately keeping it flying and finding appropriate 30 year old mission system spares etc is a bit of a headache - unless you can find a manufacturer with the capability of building antique computer equipment at a reasonable cost.