Google updated its YouTube Data API on April 20, 2015. This caused the YouTube app not to work with some older devices. All iPod touch, iPhone and iPad models running iOS 7.0 or later should not be affected by the change.
Apple devices that no longer support the YouTube iOS app after April 20, 2015:
iPod touch (1st to 4th generation)Original iPad
Owners of these devices can still access YouTube by navigating to m.youtube.com on their mobile browser such as Safari or Chrome.
The YouTube app is no longer supported by the 1st or 2nd generation Apple TV.
You can see a complete list of devices that no longer support the YouTube app here. This includes Android devices, Roku, Samsung Blu-ray players and more.