A switch has been flipped in the backend of YouTube. Users viewing videos on the Android and iOS apps of the video streaming app can now watch select clips in 60 frames per second. We say select because not everyone uploads videos at 60fps. It seems there isn’t even an app update you need to download for this to be activated.
If you’re on mobile data though, you have to be a bit wary of watching videos in 60fps and even the latest 1440p resolution that YouTube also supports. That is unless you want to max out your monthly data allocation. But if you're on Wi-Fi, you have nothing to worry about.
Source: Droid Life