Interesting that the above trailer got the dates wrong on her age. She is 19 and the deal is about when she turns 20. I just watched the film yesterday on TCM. It is a much better film that I had realized.
It is really one of the best films about Mississippi that I have ever seen. It may seem over the top but there really were people like that in Mississippi back in those days. This is a little known and less appreciated film by Williams and director Elia Kazan. It was banned in Boston as I recall.
Carroll Baker was nominated for an Oscar for her performance as Baby Doll. http://www.imdb.com/title/tt0048973/