Most of the content on YouTube is uploaded by individuals, but media corporations including CBS, the BBC, Vevo, and Hulu offer some of their material via YouTube as part of the YouTube partnership program. Unregistered users can watch, but not upload, videos on the site, while registered users are permitted to upload an unlimited number of videos and add comments to videos. Age-restricted videos are available only to registered users affirming themselves to be at least 18 years old.