Determining app compatibility and iPhones?

I’m working on an app for iPhones and android devices. I’m trying to figure which smartphones the app will run in.

In the case of the iPhone, I’ve seen that developers tend to develop the app so that there’s support up to a certain iphone (ie. Iphone 6). For example, most apps are not compatible with the iphone 4s.

If an app is a simple calculator with just a few simple square buttons, why is it that the app is designed to run in only the latest iphones?

This calculator only has a few buttons, so what does the latest iPhone have that an older one (ie. Iphone 4) does not have?