Korra is 17 years old at the start of The Legend of Korra. She is introduced as a teenage Avatar who has already mastered waterbending, earthbending, and firebending, but still needs to learn airbending.
Korra was discovered as the Avatar when she was four years old. By age 17, she had spent most of her life training under the protection of the White Lotus in the Southern Water Tribe. Her isolation was intended to keep her safe, but it also left her impatient and unfamiliar with the wider world.
The series premiere, “Welcome to Republic City,” begins when Korra leaves her secluded training compound and travels to Republic City to study airbending with Tenzin, the son of the previous Avatar, Aang. A common mix-up is confusing her starting age with later seasons: Korra is 17 in Book One, 18 in Books Two and Three, and 21 in Book Four.