Prevents you from having to sign in to view age restricted videos on YouTube
< Feedback op Youtube Age Confirmation Bypass
There isn't much that can be done about that unfortunately. The script uses the embed url of the video to bypass the age filter and embedded playback can be disabled by the uploader. In those cases you'll have to log in.
No playback when embedding isn't allowed
“This video contains content from VEVO. It is restricted from playback on certain sites.
[ Watch on YouTube ]”
Example: https://www.youtube.com/watch?v=TlO4FWrVE2I