Earlier quoted context omitted.
Apple only offers older versions of your app to users that have previously purchased it. New users will still get the traditional "this app requires iOS 7" failure message.
I believe you can purchase apps you haven't purchased before. The writer of the article I read on this tried it. I've downloaded free apps whose latest versions weren't compatible on my device, and got the older version that works for my old (iOS 3.2) device a couple of months ago.
Did you make the "purchase" from your device or from iTunes on your computer? It's never worked when I've tried from a device.