The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
17. And (knowledge belongs) to those who are bound to chastity; for in scripture (that condition of life is mentioned).
Scripture shows that knowledge is valid also for those stages of life for which chastity is prescribed. Now in their case knowledge cannot be subordinate to work because work is absent; for the works prescribed by the Veda such as the Agnihotra are no longer performed by men who have reached those stages.--But, an objection is raised, those stages of life are not even mentioned in the Veda!--This is not so, we reply. Certain Vedic passages clearly intimate them; so e.g. 'There are three branches of the law' (Kh. Up. II, 23, 1); 'Those who in the forest practise faith and austerity' (Kh. Up. V, 10, 1); 'Those who practise penance and faith in the forest' (Mu. Up. I, 10, 11); 'Wishing for that world only mendicants wander forth' (Bri. Up. IV, 4, 22); 'Let him wander forth at once from the state of studentship.'--That the stages requiring chastity are open to men whether they have reached householdership or not, and whether they have paid the debts (of procreating a son, &c.) or not, is known from scripture and Smriti. Herefrom also follows the independence of knowledge.