Hi Tiber!
For iOS : We support iOS 10 and above for the Current Account. You can use iOS 9 on prepaid for the moment, but we’ll be shuttering the prepaid programme within the next few months.
For Android : We support Android 5.0 and above, on devices with Google Play Services and the Google Play Store.
This means we don’t offer support for devices running heavily custom versions of Android that don’t use Google Play Services or the Google Play Store, such as the Kindle Fire tablets. Although it’s possible to side-load the APK file of the app on such devices, you won’t be able to get updates, and as we don’t officially provide the APK file, there are security risks to downloading it, should you find that someone has ripped and uploaded it to a file-sharing site.