Having begun his apprenticeship (with a teacher) when he was twelve years of age, Svetaketu returned to his father, when he was twenty-four, having then studied all the Vedas,—conceited, considering himself well-read, and stern.

In plain words: Shvetaketu began studying with a teacher at age twelve and came home at twenty-four. He had learned all the Vedas, but he came back proud, thinking he knew everything, and acting stiff and serious.