Downloading an old app released for an older version of iOS
Honestly I do not believe it is possible. You may have to find yourself an iOS device that already has iOS 7 on it in order to download this app.

It is also likely that even if you could download it, it just would not run as iOS has changed a lot since iOS 7.

If you have an iOS device that supports iOS 7 you may be able to jailbreak it and install an earlier version of iOS (iOS 7) from unauthorized download locations. But that is way beyond the scope of what we discuss and support here. So you would have to go elsewhere for help with that.