Loop will run on on any iPhone that is running iOS 11 or later.
Where can you find your iPhone’s iOS version? Go to your iPhone’s Settings>General>About. The iOS will be listed under “Version”
If you plan on using a 5th generation iPhone, the use of the “s” model is encouraged (iPhone 5s). iPhone 5c has an older bluetooth system and battery drain is fairly significant, making it less ideal for Loop use.