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.
I purchased them from a device. In my case, it was a first gen iPad which was still in its original OS version (3.2 I believe).