Looks like the 5S is the oldest phone the current version of the app supports. (The 5C is equivalent to the 5 in terms of specs). That said, sometimes older phones will let you install older versions of an app if there is one your phone supports. If not, then the game isn't available for that device. To check if your phone can run the game, just open the app store on your phone and see what it says when you pull up Stardew Valley.