If you're talking about a film that isn't very appropriate for children, you would probably say "that film is for adults".

Interesting question Sara! Options: 1) That film is for adults (it has been designed for them to watch and is probably inappropriate for children) 2) That is an adult film (it has been designed for them and is definitely inappropriate for children) 3) That film is adult (really only used in the USA - means the same as number 2, but it refers to it having "an adult rating"). Do these make sense? In short, any are fine! Tom

Esa pelicula es para adultos. That film is for adults. To say that is an adult film is saying the same thing only using less words. By saying it is an adult film implies it is not suitable for children.