In an effort to replace Draconian content restrictions of the Hayes Code, the Motion Picture Association of America, now just the Motion Picture Association (MPA) came up with a movie rating system in 1968 that is essentially the same to this day. The highest level was Rated-X, which at the time banned the admission of anyone under 16 years of age.
Source：https://www.cbr.com/best-nc-17-rated-movies/
