Vivien Leigh
Barbara O‘Neil (†70), who played "Ellen O‘Hara", "Scarlett's" mother in "Gone With The Wind" was just 28 years old when shooting the film, making her only three years older than her on-screen daughter Vivien Leigh, who was 25 at the time. Vivien's character "Scarlett" is supposedly 16 years old at the beginning of the epic masterpiece!