Trying to get YouTube running on a device with iOS 7.1.2, like an iPhone 4, can be a bit of a challenge. Because Google no longer officially supports such an old operating system, the modern app won't even show up in your App Store updates.
Even if you download a legacy IPA, you may see an "Error Loading" or "Update Required" message because YouTube's old APIs are shut down. You can fix this with TubeRepair Option A: TubeRepair Tweak and add the repository: Youtube Ipa For Ios 7.1 2 Download
If you have previously downloaded YouTube on your Apple ID with a newer device, you can often download a compatible version directly on your old device. Open the App Store on your iOS 7.1.2 device. in the list and tap the cloud icon. Trying to get YouTube running on a device with iOS 7
Running the YouTube app on iOS 7.1.2 (commonly used on the iPhone 4) is challenging because the official App Store no longer supports this version You can fix this with TubeRepair Option A: