iOS: To install Smart24, you need to have iOS 10 or higher, which means you need to have an iPhone 5 or higher.
Android: To install Smart24, you need Android version 4.4 or above ("Kitkat", first released in October 31, 2013)
As a general piece of advice, we recommend that you always use the latest operating system provided by Apple or by your Android phone vendor. This helps to keep your phone secure and makes sure you have the latest features.
On an iPhone you can check if you have the latest version of iOS by tapping on the Settings app and then General -> Software Update.
If you previously installed versions of Smart24 prior to version 4.0 on iOS9, or Android 4.1-4.3, these versions will continue to work, but will be eventually 'deprecated' some time in 2019-2020.