It often occurs to me that two divine children demonstrate their purpose of descent to the earth through their very upanayana samskara. One of the two is Sankara who gave new life to the Vedic religion and the other is Jnanasambhandhar. The Tamil Periyapuranam mentions that Jnanasambhandhar's upanayana was performed when he was a small child. From the fact that he calls himself "Nanmarai Jnanasambandhan" we know that he was conversant with all the four vedas. Both the Acarya and Jnanasambandhar had to listen only once to grasp their lessons: that is why they were called "ekasantagrahins". So they could learn the vedas in one or two years. Indeed, there was no need to teach them, for they were capable of learning everything on their own. The one (Sankara) was an incarnation of Siva and the other of Subrahmanya. If they had accomplished their respective missions only after their upanayana had been performed and after they had received instruction in Gayatri, it was only to show ordinary people like us the importance of that samskara.
There was a poet called Ramabadhra Dikshita who spoke about himself thus" Before Gayatri came to me, Sarasvati was already there with me." Before he was eight years old the usual age for imparting the Gayatri mantra, he had began to compose poems. Jnanasambandar too sang his composition "Todudaya ceviyan" when he was only three years old. He had already been performing miracles. But even so he had his upanayana ceremony performed and was taught the Gayatri Mantra. If he did so it was to show the world that he was able to accomplish his great mission in life only by upholding the Vedic dharma. He has opened our eyes to the significance of the upanayana samskara and the Gayatri Mantra.
For a general background, please see here