Anna is 18 years old when Disney’s Frozen begins.
The film opens on the day of Elsa’s coronation, after the sisters have spent years separated inside the castle. Anna is eighteen at the start of the main story, while Elsa is twenty-one. Their ages are part of the character information associated with the film and help establish that both sisters are young adults when the kingdom’s gates finally reopen.
The age detail is easy to confuse with later material. Anna is younger than Elsa by three years, and Frozen II takes place three years after the first film, when Anna is therefore about twenty-one. Flashbacks in Frozen show the sisters at much younger ages, including the childhood scenes in which they build Olaf.
Anna’s age also places her sudden engagement to Hans and her decision to pursue Elsa in the context of a sheltered young royal experiencing the outside world for the first time. The story does not present her as a child during the main events.