The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
19. (The other stage of life) is to be accomplished, (according to) Bâdarâyana; on account of the scriptural statement of equality.
The teacher Bâdarâyana is of opinion that that other stage of life is something to be accomplished. The view that there is a contradiction because the other stage of life is stated in the Veda and, on the other hand, works such as the Agnihotra must necessarily be performed, and that, in order to remove this contradiction, that other stage of life must be entered upon by those only who are not qualified for active worship, he rejects; being of opinion that that other stage is to be entered upon, in the same way as the state of the householder, even by him who does not wish to do so.--On what ground?--'On account of the scriptural statement of equality.' For we have a passage (viz. 'There are three branches of the law,' &c.) which refers equally to that other stage as to the state of the householder. As the state of the householder which
is enjoined in other passages only is here referred to, so also that other stage of life. The case is analogous to the reference made to the wearing of the sacrificial thread round the neck or on the right shoulder--which two modes are established in other scriptural passages--in a passage the purpose of which it is to enjoin the wearing of the thread on the left shoulder. The other stage must therefore be entered upon in the same way as the state of the householder.--Analogously in the passage, 'Wishing for that world only mendicants leave their homes,' the last stage of life is mentioned together with the study of the Veda, sacrifice and so on, and in the passage, 'Those who in the forest,' &c., with the knowledge of the five fires.--The remark, made above by the pûrvapakshin, that in such passages as 'austerity is the second' there is unambiguous reference to a further stage of life, is without force, since there is a reason enabling us to determine what is meant. The text proclaims in the beginning that there are three subdivisions ('There are three branches of the law'). Now the sacrifice and the other duties (which the text enumerates subsequently to the introductory clause) can, because they are more than three, and rest on separate originative injunctions, be comprised within the three branches only if they are connected with one of the stages of life. Now the terms 'sacrifice' and so on indicate that the stage of householdership constitutes one branch of the law, and the term 'Brahmakârin' clearly denotes another stage; what then remains but to assume that the term 'austerity' also denotes a stage of life, viz. the one in which austerity is the chief thing? Analogously the reference to the forest--in the passage, 'Those who in the forest,'--indicates that by the austerity and faith mentioned there we have to understand that stage of life in which austerity and faith are the chief thing.--From all this it follows that the further stage of life has to be gone through, even if the passage under discussion should do nothing but refer to it.