Shortcuts app is available for all the devices running iOS 12. You can use this App Store link to access the app.
Open this answer directly on your iPhone/iPad and tap on the App Store link above to directly go to the App Store page of the Shortcuts app and download it. Or if you are using Safari on macOS, open the App Store link in a new tab and AirDrop it to your iPhone/iPad. This will directly open the App Store page for the app where can download it.
Your devices aren't too old to run the app. There is no special device capabilities required for running the app. The app is supported on all the devices capable of running iOS 12. (For iPhones that goes as far back as iPhone 5s.)
This app used to be called Workflow and was a third party app when apple aqui-hired them and the app. Some great writing on how to use the app in practical settings is at https://www.macstories.net/
Apple also made a pretty in dept web site to explain how to use this big step forward for iOS - the features are baked deeply into iOS and third party developers can write extensions that let shortcuts and siri reach into those apps and do work and automation.