Even after the download, iOS 9 can struggle with video playback.
Double-click the Home button and close all other apps. youtube for ios 9.3.5 download
If you have never "purchased" YouTube on your Apple ID before, this won't work. To fix this, sign into your Apple ID on a newer iPhone or a computer with iTunes, "buy" the free YouTube app there, then return to your iOS 9 device. Method 2: The Safari Web App Shortcut Even after the download, iOS 9 can struggle