I know SunSally and a couple others can answer this best. But my beds, cira 2001, are worth more than half the original price. I am not sure where they got those numbers from, but if a 3 year old bed is really only worth .15 to .20 cents on the dollar no one would by new!
As for whats considered old, maybe something from 10 years ago. I see plenty of beds that, like mine, are 5 to 10 years old and in service. Its less about age and more about the type of bed and brand that can determine value.
Here is where I go to "bluebook" a bed:
http://www.2ndsuntan.com