The project revolves around a thirtysomething author of young adult novels who returns to her hometown and begins to follow her now-married ex-boyfriend.
Screenwriter Cody, who recently gave birth to son Macello Daniel Maurio, won an Oscar for her Juno screenplay in 2008. The movie was also directed by Reitman, who will helm and executive produce Young Adult.
The pair's new project is being eyed to begin production in New York in November, reports Deadline.
Theron is also expected to star in George Miller's Mad Max revamp Fury Road next year.