CHĀNDOGYA UPANISHAD · 6.1.2

स ह द्वादशवर्ष उपेत्य चतुर्विंशतिवर्षः सर्वान् वेदान् अधीत्य महामना अनूचानमानी स्तब्ध एयाय ।
तं ह पितोवाच ॥

sa ha dvādaśavarṣa upetya caturviṃśativarṣaḥ sarvān vedān adhītya mahāmanā anūcānamānī stabdha eyāya ।
taṃ ha pitovāca ॥

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.

F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)