Upanayana in the Sūtras and the Smṛtis
It was in the Sūtra period that the upanayana saṃskāra seems to have been fully established.[1] Most of the details of the ceremony are laid out in the gṛhya-sūtras. The Dharma-sūtras and Smṛtis have nothing new to say about the ritualistic aspects apart from what has already been said earlier; they primarily develop the social side of the saṃskāra. It was also perhaps during this period that the Upanayana became compulsory for men from the first three varṇas.