This guide demonstrates how to use the App Store's purchase history to bypass compatibility errors without needing a computer:
However, there is a massive problem:
Apple stopped supporting iOS 9 years ago. If you pull an iPhone 4S, iPad 2, or iPad 3 out of a drawer today and open the stock YouTube app, you get a cruel message: "This app needs to be updated." Youtube.ipa For Ios 9.3.5
If you really want to use an old iOS device as a YouTube player, jailbreaking iOS 9.3.5 is the most practical route. This guide demonstrates how to use the App
Now, you should be able to launch YouTube. Youtube.ipa For Ios 9.3.5