You will need to install a user script manager extension to install this script.
At YouTube, some videos are age-restricted. For example this video. Normally you need a login to watch the video. This script uses the embed feature of YouTube, which let watchers watch age-restricted videos without login.
If you miss the related videos, you can also install the script Youtube: Show related videos if missing.
Successfully tested with:
Don't have a userscript manager like Greasemonkey, Tampermonkey or Violentmonkey installed? You can also install the userscript as a bookmarklet.