This is how it works:
You have 2 versions of Pages on your Mac.
Pages 5 is in your Applications folder.
Pages '09/'08 is in your Applications/iWork folder.
You are alternately opening the wrong versions.
Pages '09/'08 can not open Pages 5 files and you will get the warning that you need a newer version.
Pages 5.01 can not open Pages 5.1 files and you will get the warning that you need a newer version.
Pages 5 can open Pages '09 files but may damage/alter them. It can not open Pages '08 files at all.
Once opened and saved in Pages 5 the Pages '09 files can not be opened in Pages '09.
Once opened and saved in Pages 5.1 files can not be opened in Pages 5.
Anything that is saved to iCloud is also converted to Pages 5 files.
All Pages files no matter what version and incompatibility have the same extension .pages.
Pages 5 files are now only compatible with themselves on a very restricted set of hardware, software and Operating Systems and will not transfer correctly on any other server software than iCloud.
Apple has removed almost 100 features from Pages 5 and added many bugs:
Archive/trash Pages 5, after exporting all Pages 5 files to Pages '09 or Word .docx, and rate/review it in the App Store, then get back to work.