Anne Hathaway was about 17 or 18 years old when she played the role of Mia Thermopolis in "The Princess Diaries." The film was released on August 3, 2001, and Hathaway herself mentioned in 2021 on the 20th anniversary that she was just 18 when she played the part. There are also references stating she was 17 when she landed the role and during the audition period shortly before filming started.