Robert Redford was born on August 18, 1936. The movie "Indecent Proposal" was released in 1993, and at that time, Robert Redford was 56 years old. This age aligns with the age gap noted in the film between Robert Redford and Demi Moore's characters, where Redford was 55-56 during filming. Thus, Robert Redford was about 56 years old when he starred in "Indecent Proposal".