The issue is that current releases (e.g. Pages for iOS, Pages for iCloud, and Pages for Mac ) include the version of the oldest compatible version of Pages that can open the current document, and unsupported Mac, and iOS hardware platforms that the document cannot be opened on.
So for Pages v7.3, it shows that the oldest version of Pages for Mac that can open its document is Pages v5.6. But is also lists the following hardware as unsupported:

Your iMac is an early-2009 model (iMac9,1), and fails the supported models list above, so even though you might have Pages v5.6 or later on your iMac, it fails the unsupported model test, and Pages won't open the document.
If you were using any other non-Apple, cross-platform, word processing solution (e.g MS Word), then hardware obsolescence of documents would likely not occur.