Google has made changes to its YouTube application that breaks compatibility with certain devices.
Specifically, Google updated the YouTube Data API so it can incorporate new features. The changes in the API make YouTube non-functional on iPhones from 2012 and earlier, as well as video-based hardware, such as the Apple TV, versions 1 and 2 of Google TV, and select smart TVs dating earlier than 2013.
According to Google, iPhone owners running iOS 7 and up should not experience any trouble. If they do, downloading the current version of the app should resolve any problems.
People who own newer versions of the Apple TV, newer smart TVs, and the Nexus Player should have no trouble using YouTube. The change went into effect April 20.