Hays Code

Hays code are a set of industry censorship guidelines which governed the production of the vast majority of United States motion pictures released by major studios from 1930 to 1968.
Hays code were the first attempt at introducing film censorship in the US through laying down a series of guidelines to film producers.
Nakedness, suggestive dances, homosexuality & cruelty to children were some of the things prohibited.

The code was abandoned in 1967 & in 1968 it was replaced by the rating system that we now have. Nobody under 18 years of age is allowed into an R-rated movie (unless accompanied by an adult).