The answer is less a matter of physical age and more about what kind of software the phone can run and whether the app was compiled for an operating system compatible with the phone. In that respect, iPhones and Android phones differ greatly.
Apple supports its smartphones for about five years after a model is released, giving those devices the ability to run the latest iOS versions their patches. For example, if you are using an iPhone 5s , it might be difficult to upgrade to iOS 13, given that it is the oldest phone which can run iOS 12 – released in September 2018. You can bet the farm a 5s won’t run iOS 13.
Gauging an Android phone’s limits can be harder because they’re not as standardized as iPhones.
The product cycle on Android phones is less consistent than on iPhones. There are hundreds of smartphone makers which use Android. Accordingly, it’s less than certain whether an old Samsung handset will run the latest OS after even three years.
Why does this matter? When a developers build apps, they use software development kits (SDK) built for the current operating system. As your old phone slips further behind the current OS, the SDKs developers use will no longer work on your device. One or two operating systems behind may not make that much of a difference, but eventually you’ll run into issues and Apple or Google will stop your phone from downloading new apps entirely.
Old phones also look terrible on new apps because they lack the resolution utilized by new phones. Developers build apps for new phones, not old ones. The difference is significant. Going back to the 5s – released in 2013 – which runs at 640×1136 ; that’s half the resolution of today’s iPhone XS which runs at 2688 x 1242.
There is a silver lining. Most people replace their phones every two or three years. So despite the fact your old phone can’t run your new app or looks terrible doing it, rest easy knowing that most people are not trying to run your app from a device built in the late cretaceous.