Vivien Lyra Blair is nine years old, which, given that Kenobi takes place ten years after the events of Revenge of the Sith, puts her near to Princess Leia’s true age at the time.
The Kenobi series takes place in 9 BBY (Before the Battle of Yavin), with Leia being born after Anakin Skywalker’s conversion to the Dark Side in the year 19 BBY (Before the Battle of Yavin). This puts the actor’s real-life age very close to the character she’s portraying.
That’s also the same age as Anakin Skywalker in Star Wars Episode I: The Phantom Menace, who was played by Jake Lloyd, who was also the same age as the character.
While it is common for films and television series to cast slightly older actors to play younger characters, Star Wars has a history of ignoring the pattern and casting actors who are roughly the same age as their characters.
Vivien Lyra Blair doesn’t have a lot of on-screen credits, which is understandable given her age.
Prior to Kenobi, she was best known for acting alongside Sandra Bullock in the 2018 thriller Bird Box, which went on to become Netflix’s most-watched film.
Aside from that, she’s starred in the family-friendly 2020 feature We Can Be Heroes (with fellow Star Wars actor Pedro Pascal) and Joseph Gordon-Apple Levitt’s TV comedy Mr. Corman.