Apple is not really into backward compatibility. When they release a new version of Pages or Numbers, it will open files from the previous version. Generally speaking, you can’t go back 2 or 3 versions. This means that every time you upgrade iWork, you’re really supposed to find all of your old files and upgrade them.
I’m not going to do that.
As ugly as it is, Office is big on backward compatibility. You can still open really, really old Word or Excel files in recent editions of Office.