Sally Nash and Joe Therrian are a Hollywood couple celebrating their sixth wedding anniversary shortly after reconciling following a period of separation. He is a novelist who is about to direct the screen adaptation of his most recent bestseller; she is an actress he has opted not to cast in the lead role, despite the fact it's partly based on her, because he feels she's too old for the part. This decision, coupled with an ongoing dispute about their barking dog Otis with their strait-laced, non-industry neighbors, clean-and-sober writer Ryan and interior decorator Monica Rose, has resulted in an undercurrent of tension between the two as they prepare for the arrival of their guests.
Among them are aging actor Cal Gold, Sally's co-star in the romantic comedy film she presently is shooting, his wife Sophia, and their two young children; director Mac Forsyth, who is helming Sally and Cal's film, and his anorexic, neurotic wife Clair; photographer Gina Taylor, whose relationship with Joe prior to his marriage and ongoing close friendship since troubles Sally; business manager Jerry Adams and his wife Judy; eccentric violinist Levi Panes; Jeffrey, Joe's roommate - and lover - at Oxford; and up-and-coming actress Skye Davidson, whom Joe has cast in the role Sally believes deservedly is hers. In an effort to dispel the simmering animosity between them and their neighbors, Sally and Joe have invited the Roses as well.
The early part of the evening is devoted to charades and lighthearted entertainment. Following a series of toasts offered by the guests, Joe distributes the ecstasy Skye brought them as a gift. As it begins to take effect, the night deteriorates, accusations are made, secrets are revealed, and relationships slowly unravel. Complicating emotions triggered by the drug are the disappearance of Otis and a phone call from Joe's father bringing tragic news about his beloved sister Lucy.

Question: Why won't Joe cast Sally in his new project?
Answer: He feels she's too old for the part
Alternative answer: She's too old.