The changes are set to take effect today.
As we upgrade the YouTube Data API to bring more features, we’ll begin shutting down the old version on April 20, 2015. This will result in the current YouTube app not working on certain device models from 2012 and older. We will continue to focus our efforts on improving our official YouTube app for TV which is available on most 2013 and newer Smart TVs and Blu-ray players, game consoles and streaming devices.
If you are using an iOS 6 device or lower, fortunately, you can still access YouTube from the browser. If you have a second generation Apple TV, "unfortunately there’s no current way to watch YouTube on these devices."