Srimad Bhagavatam Canto 11, Chapter 17, Text 22

SB 11.17.22

dvitiyam prapyanupurvyaj
 janmopanayanam dvijah
vasan gurukule danto
 brahmadhiyita cahutah
The twice-born member of society achieves second birth through the sequence of purificatory ceremonies culminating in Gayatri initiation. Being summoned by the spiritual master, he should reside within the guru’s asrama and with a self-controlled mind carefully study the Vedic literature.
The term dvija, or “twice-born,” here indicates the three superior classes, namely brahmanas, ksatriyas and vaisyas, who all receive the Gayatri 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 brahmana boy, if qualified, may be initiated with Gayatri mantra at the age of twelve, and ksatriyas and vaisyas a few years later. In order to become enlightened with spiritual knowledge, the boy resides within the gurukula, or asrama 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 anupurvyat in this verse indicates the system of samskaras, or purificatory rites, beginning with garbhadhana-samskara, or the purification of the sexual act. Generally, sudras 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.
Srimad Bhagavatam Canto 11, Chapter 17, Text 21
Srimad Bhagavatam Canto 11, Chapter 17, Text 23