SB 11.17.22

dvitīyaṁ prāpyānupūrvyāj
 janmopanayanaṁ dvijaḥ
vasan gurukule dānto
 brahmādhīyīta cāhūtaḥ
dvitīyam — second; prāpya — achieving; ānupūrvyāt — by the gradual process of purificatory ceremonies; janma — birth; upanayanam — Gāyatrī initiation; dvijaḥ — a twice-born member of society; vasan — residing; gurukule — in the āśrama of the spiritual master; dāntaḥ — self-controlled; brahma — the Vedic literatures; adhīyīta — should study; ca — and also understand; āhūtaḥ — being summoned by the spiritual master.
The twice-born member of society achieves second birth through the sequence of purificatory ceremonies culminating in Gāyatrī initiation. Being summoned by the spiritual master, he should reside within the guru’s āśrama and with a self-controlled mind carefully study the Vedic literature.

The term dvija, or “twice-born,” here indicates the three superior classes, namely brāhmaṇas, kṣatriyas and vaiśyas, who all receive the Gāyatrī mantra, which signifies their second birth through spiritual initiation. One’s first birth is biological, or seminal, and does not necessarily indicate that one is intelligent or enlightened. A young brāhmaṇa boy, if qualified, may be initiated with Gāyatrī mantra at the age of twelve, and kṣatriyas and vaiśyas a few years later. In order to become enlightened with spiritual knowledge, the boy resides within the gurukula, or āśrama of the spiritual master. The International Society for Krishna Consciousness has established similar gurukulas all over the world and is issuing a great call to civilized human beings to arrange for the proper education of their children. Every young boy and girl should learn to be self-controlled and should become enlightened through study of authorized Vedic literatures. In this way, unlike ordinary animals, insects, fish and birds, etc., an enlightened human being may take birth twice and thus become perfect in the knowledge that leads to ultimate liberation. The word ānupūrvyāt in this verse indicates the system of saṁskāras, or purificatory rites, beginning with garbhādhāna-saṁskāra, or the purification of the sexual act. Generally, śūdras and those who do not follow the Vedic system are not attracted to such purificatory ceremonies; therefore they remain ignorant of spiritual life and envious of the bona fide spiritual master. Those whose character has been civilized by a systematic cleansing process give up the tendency to be argumentative and whimsical and instead become submissive and eager to learn in the presence of a bona fide spiritual master.