Apple Watch Series 1 and iPhone 5s
Is it possible to pair an Apple Watch Series 1 to an iPhone 5s on iOS 12.1.2? I don’t know what version the series 1 is on. Let’s assume it’s on the highest version it can run. Will it pair?
Apple Watch
Just yesterday I helped somebody else with a similar iPhone 5s and watch question.
Give it a read and see if it will help you:
What Apple Watches can pair with an iPhon… - Apple Community
Edit - I'm leaning towards, "Yes"