Electronics' reliability seems to follow a 'bathtub'-looking curve, in general - relatively high probability of failure at first (due to possible component and assembly faults that tend to show up early), followed by (hopefully) very few issues, and eventually an increasing probability of failure as individual components reach the end of their lives. If you're looking at something that was purchased in 2005 and made it through without issues(*), then I'd assume that you should be set. (This is assuming that we can guess how your device will behave based on how much I know about most devices behaving... but this is a decent enough assumption for common electronics, and apart from things like tube and transformer pooch-screwing we can assume this to be the case.) 1995 may be a different issue, and 1985 may warrant a decent inspection by a trusted technician... the idea is that you want to look at the lifespan of similar devices that didn't bite the dust very early on (a few years of use, in this case) and make sure that you're fine with spending the asking price on a device that may only last (expected lifetime - current age) or so.
(*) I'm assuming that the device has been properly maintained and that all 'real' issues have led to a trip to a local technician. You may be bothered by an issue that the previous owner(s) thought was normal, or you may soon see the effects of past neglect and abuse, but assuming a decently-maintained and carefully-evaluated device, I would expect few problems.
This is a remarkably useless answer if you're used to screwing with electronics, but if not then it may at least guide your rough guesses. More specific advice can be given with more specific information, so consider giving details whenever possible.
Jeff
