This is groudspeaks answer. I have an iPhone 5c
Thank you for contacting Geocaching HQ.
As technology advances, so does the free Geocaching® app The new app will not work in older devices. Compatibility for each phone and app versions:
Free iPhone Geocaching® App
Free Android Geocaching® App
Retired Geocaching Classic iPhone app
Retired Geocaching Classic Android app
Please note we are only updating the free Geocaching® app from this point forward. The Classic app will have access to Geocaching functionality through September 2016. However, updates to your phone may cause the app's features to stop working. We can't guarantee the Classic app will work if you update your device after June 30, 2016.