The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut [1890] at sacred-texts.com
36. On account of the reference to ceremonial purifications (in the case of the higher castes) and on account of their absence being declared (in the case of the Sûdras).
That the Sûdras are not qualified, follows from that circumstance also that in different places of the vidyâs such ceremonies as the upanayana and the like are referred to. Compare, for instance, Sat. Br. XI, 5, 3, 13, 'He initiated him as a pupil;' Kh. Up. VII, 1, 1, 'Teach me, Sir! thus he approached him;' Pra. Up. I, 1, 'Devoted to Brahman, firm in Brahman, seeking for the highest Brahman they, carrying fuel in their hands, approached the venerable Pippalâda, thinking that he would teach them all that.'--Thus the following passage also, 'He without having made them undergo the upanayana (said) to them' (Kh. Up. V, 11, 7), shows that the upanayana is a well-established ceremony 1.--With reference to the Sûdras, on the other hand, the absence of ceremonies is frequently mentioned; so, for instance, Manu X, 4, where they are spoken of as 'once born' only ('the Sûdra is the fourth caste, once-born'), and Manu X, 126, 'In the Sûdra there is not any sin, and he is not fit for any ceremony.'
227:1 The king Asvapati receives some Brâhmanas as his pupils without insisting on the upanayana. This express statement of the upanayana having been omitted in a certain case shows it to be the general rule.