The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
38. And the promotion (of knowledge is bestowed on them) through special acts.
Also for widowers, &c., the favour of knowledge is possible through special acts of duty, such as praying, fasting, propitiation of divinities, &c., which are not opposed to their âsrama-less condition and may be performed by any man as such. Thus Smriti says, 'By mere prayer no doubt the Brâhmana perfects himself. May he perform other works or not, the kindhearted one is called Brâhmana (Manu Samh. II, 87), which passage shows that where the works of the âsramas are not possible prayer qualifies for knowledge. Moreover knowledge may be promoted by âsrama works performed in previous births. Thus Smriti also declares, 'Perfected by many births he finally goes the highest way' (Bha. Gîtâ VI, 45); which passage shows that the aggregate of the different purificatory ceremonies performed in former births promotes knowledge.--Moreover knowledge--as having a seen result (viz. the removal of ignorance)--qualifies any one who is desirous of it for learning and so on, through the mere absence of obstacles 1. Hence there is no contradiction in admitting qualification for knowledge on the part of widowers and the like.
316:1 I.e. any one who wishes to learn may do so, if only there is no obstacle in the way. No special injunction is wanted.