The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
40. But of him who has become that (i.e. entered on a higher âsrama) there is no becoming not that (i.e. descending to a lower one), according to Gaimini also, on account of restrictive rule, absence of such like (i.e. statements of descent), and non-existence (of good custom).
It has been established that there are stages of life for which chastity is obligatory. A doubt here arises whether one who has entered them may for some reason or other fall from them or not.--The pûrvapakshin maintains that as there is no difference a person may descend to a lower stage, either from the wish of well performing the duties of that stage, or influenced by passion and the like.--To this we reply as follows, 'Of him who has become that,' i.e. of him who has reached the stages for which chastity is obligatory, there is no 'becoming not. that,' i.e. descending thence.--Why?--'On account of restrictive rule, absence of such like, and non-existence.' That means: there are, in the first place, restrictive rules declaring that a descent may not take place. Compare 'for life mortifying the body in the house of a tutor' (Kh. Up. II, 23. 2); 'He is to go into the forest, that is he is not to return thence, that is the Upanishad;' 'Having been dismissed by the teacher he is to follow one of the four âsramas, according to rule, up to release from the body.'--In the second place there are texts teaching the ascent to higher âsramas ('Having completed the Brahmakarya state he is to become a householder; he may wander forth from the Brahmakarya state'); but there are none teaching the descent to lower âsramas.--And in the third place there exists no good custom of that kind.--The descent to a lower âsrama can in no way be based on the wish of well performing the duties of that âsrama; for
[paragraph continues] Smriti says, 'One's own duty, however badly performed, is better than another duty well carried out' (Bha. Gîtâ III, 35). And the principle is that whatever is enjoined on a certain person constitutes his duty, not what a person is able to perform well; for all duty is characterised by injunction. Nor is a descent allowed owing to the influence of passion, &c.; for restrictive rules are weightier than passion.--By the word 'also' the Sûtrakâra indicates the consensus of Gaimini and Bâdarâyana on this point, in order to confirm thereby the view adopted.