Running an app build with very old iOS SDK on a device running current iOS I have 2 iOS devices, an iPhone 6s, and an iPhone 4. I'm currently using OS X Mavericks 10.9.0 and plan on using Xcode.

The iPhone 6s is running iOS 12.2, and iPhone 4 is running iOS 7. My Xcode version is 5.1.0.

Will I be able to deploy an app using Xcode 5 on my iPhone 6s?
Xcode 5.1.0 include iOS 7.1 SDK. You cannot debug an app build with this Xcode/iOS SDK combination on your iPhone 6s running iOS 12.2 (iOS 12 was non-existent when Xcode 5.1 was released and Xcode is not aware of it).

You can however, in-theory generate the app IPA and deploy it on iOS 12. iOS 11 and later require your app to have executable support for 64-bit mode. So you need to make sure to build your iOS app with 64-bit mode.

However, you may end up hitting roadblocks as iOS 7 is ancient in terms of how iOS has evolved. With a decently complex app, you may run into issues when running an app built with much older SDK on a device running current iOS.