The whole purpose of Vedic civilization and of reading the Vedas is to attain the perfect stage of devotional service in the human form of life. According to the Vedic system, therefore, from the very beginning of life the brahmacarya system is introduced so that from one's very childhood—from the age of five years—one can practice modifying one's human activities so as to engage perfectly in devotional service. As confirmed in Bhagavad-gītā (2.40), svalpam apy asya dharmasya trāyate mahato bhayāt: "Even a little advancement on this path can protect one from the most dangerous type of fear." Modern civilization, not referring to the verdicts of Vedic literature, is so cruel to the members of human society that instead of teaching children to become brahmacārīs, it teaches mothers to kill their children even in the womb, on the plea of curbing the increase of population.
(Srimad Bhagavatam----7:6:1----purport).

Before entering household life, one should be trained as a brahmacārī, living under the care of the guru, whose place is known as the guru-kula. Brahmacārī guru-kule vasan dānto guror hitam (S.B.7:12:1). From the very beginning, a brahmacārī is trained to sacrifice everything for the benefit of the guru. A brahmacārī is advised to go begging alms door to door, addressing all women as mother, and whatever he collects goes to the benefit of the guru. In this way he learns how to control his senses and sacrifice everything for the guru. When he is fully trained, if he likes he is allowed to marry. Thus he is not an ordinary gṛhastha who has learned only how to satisfy his senses. A trained gṛhastha can gradually give up household life and go to the forest to become increasingly enlightened in spiritual life and at last take sannyāsa. Prahlāda Mahārāja explained to his father that to be freed from all material anxieties one should go to the forest. Hitvātma-pātaṁ gṛham andha-kūpam. One should give up his household, which is a place for going further and further down into the darkest regions of material existence. The first advice, therefore, is that one must give up household life (gṛham andha-kūpam). However, if one prefers to remain in the dark well of household life because of uncontrolled senses, he becomes increasingly entangled by ropes of affection for his wife, children, servants, house, money and so on. Such a person cannot attain liberation from material bondage. Therefore children should be taught from the very beginning of life to be first-class brahmacārīs. Then it will be possible for them to give up household life in the future.
(Srimad Bhagavatam----7:6:9----purport).

This chapter particularly describes the brahmacārī and the person in the vānaprastha stage, and it also gives a general description of the four āśramas-brahmacarya, gṛhastha, vānaprastha and sannyāsa. In the previous chapter, the great saint Nārada Muni has described the varṇa institution of society, and now, in this chapter, he will describe the stages of spiritual advancement in the four āśramas, which are known as brahmacarya, gṛhastha, vānaprastha and sannyāsa.
The brahmacāri should live under the care of the true spiritual master, giving him sincere respect and obeisances, acting as his menial servant, and always carrying out his order. Thebrahmacārī should engage himself in spiritual activities and study the Vedic literature under the direction of the spiritual master. According to the brahmacarya system, he should dress with a belt, deerskin, and matted hair and should bear a daṇḍa, waterpot and sacred thread. He should collect alms daily in the morning, and in the evening whatever alms he has collected he should offer to the spiritual master. A brahmacārī should accept prasāda upon the order of the spiritual master, and if the spiritual master sometimes forgets to order the disciple to eat, the disciple should not take prasāda on his own initiative; rather, he should fast. The brahmacārī should be trained to be satisfied with eating what is absolutely necessary, he should be very expert in executing responsibilities, he should be faithful, and he should control his senses and try to avoid the association of women as far as possible. A brahmacārī should very strictly abstain from living with women and should not meet with gṛhasthas and those too addicted to women. Nor should a brahmacārī speak in a lonely place with a woman.
After completing one's education as a brahmacārī in this way, one should give dakṣiṇā, an offering of gratitude, to one's guru, and then one may leave for home and accept the next āśrama-the gṛhastha-āśrama-or else one may continue in the brahmacarya-āśrama without adulteration. The duties for the gṛhastha-āśrama and brahmacarya-āśrama, as well as the duties for sannyāsīs, are prescribed in the śāstras. A gṛhastha is not meant to enjoy sex life without restriction. Indeed, the whole purpose of Vedic life is to become free from sexual indulgence. All the āśramas are recognized for spiritual progress, and therefore although the gṛhastha-āśrama gives a kind of license for sex life for a certain time, it does not allow unrestricted sex life. Therefore, in gṛhastha life also, there is no illicit sex. A gṛhastha should not accept a woman for sexual enjoyment. Wasting semen is also illicit sex.
(Srimad Bhagavatam---7:12---summary).

Brahmacarya essentially means the vow not to marry but to observe strict celibacy (bṛhad-vrata). A brahmacārī or sannyāsī should avoid talking with women or reading literature concerning talks between man and woman. The injunction restricting association with women is the basic principle of spiritual life. Associating or talking with women is never advised in any of the Vedic literatures. The entire Vedic system teaches one to avoid sex life so that one may gradually progress from brahmacarya to gṛhastha, from gṛhastha to vānaprastha, and from vānaprastha to sannyāsa and thus give up material enjoyment, which is the original cause of bondage to this material world. The word bṛhad-vrata refers to one who has decided not to marry, or in other words, not to indulge in sex life throughout his entire life.
(Srimad Bhagavatam---7:12:7----purport).

It is sometimes misunderstood that a gṛhastha, a householder, is permitted to indulge in sex at any time. This is a wrong conception of gṛhastha life. In spiritual life, whether one is a gṛhastha, vānaprastha, sannyāsī or brahmacārī, everyone is under the control of the spiritual master. For brahmacārīs and sannyāsīs there are strong restrictions on sexual indulgence. Similarly, there are strong restrictions for gṛhasthas. Gṛhasthas should indulge in sex life only in accordance with the order of the guru. Therefore it is mentioned here that one must follow the orders of the spiritual master (guru-vṛttir vikalpena). When the spiritual master orders, the gṛhastha may accept sex life. This is confirmed in Bhagavad-gītā (7.11). Dharmāviruddho bhūteṣu kāmo 'smi: indulgence in sex life without disobedience to the religious rules and regulations constitutes a religious principle. The gṛhastha is allowed to indulge in sex life during the period favorable for procreation and in accordance with the spiritual master's order. If the spiritual master's orders allow a gṛhastha to engage in sex life at a particular time, then the gṛhastha may do so; otherwise, if the spiritual master orders against it, the gṛhastha should abstain. The gṛhastha must obtain permission from the spiritual master to observe the ritualistic ceremony of garbhādhāna-saṁskāra. Then he may approach his wife to beget children, otherwise not. A brāhmaṇa generally remains a brahmacārī throughout his entire life, but although some brāhmaṇas become gṛhasthas and indulge in sex life, they do so under the complete control of the spiritual master.
(Srimad Bhagavatam---7:12:11----purport).

There are two kinds of brahmacārīs. One may return home, marry and become a householder, whereas the other, known as bṛhad-vrata, takes a vow to remain abrahmacārī perpetually. The bṛhad-vrata brahmacārī does not return from the place of the spiritual master; he stays there, and later he directly takes sannyāsa. Because Nābhāga did not return from the place of his spiritual master, his brothers thought that he had taken bṛhadvrata-brahmacarya. Therefore, they did not preserve his share, and when he returned they gave him their father as his share.
(Srimad Bhagavatam---9:4:1----purport).

If this injunction is followed, then all other rules and regulations will automatically fall into line. All other rules and regulations should be treated as assistants or servants to this one basic principle. The injunctions of rules and regulations and the resultant reactions are mentioned in the Eleventh Canto of Śrīmad-Bhāgavatam, Fifth Chapter, verses 2 and 3. Camasa Muni, one of the nine sages who came to instruct King Nimi, addressed the King and said, "The four social orders, namely the brāhmaṇas, the kṣatriyas, the vaiśyas and the śūdras, have come out of the different parts of the universal form of the Supreme Lord as follows: the brāhmaṇas have come out from the head, the kṣatriyas have come out from the arms, the vaiśyas have come out from the waist, and the śūdras have come out from the legs. Similarly, the sannyāsīs have come out from the head, the vānaprasthas from the arms, the gṛhasthas from the waist and the brahmacārīs from the legs."
(Nectar of Devotion).

When a person realizes himself to be an eternal servitor of Kṛṣṇa, he loses interest in everything but Kṛṣṇa's service. Always thinking of Kṛṣṇa, devising means by which to spread the holy name of Kṛṣṇa, he understands that his only business is in spreading the Kṛṣṇa consciousness movement all over the world. Such a person is to be recognized as an uttama-adhikārī, and his association should be immediately accepted according to the six processes (dadāti pratigṛhṇāti, etc.). Indeed, the advanced uttama-adhikārī Vaiṣṇava devotee should be accepted as a spiritual master. Everything one possesses should be offered to him, for it is enjoined that one should deliver whatever he has to the spiritual master. The brahmacārī in particular is supposed to beg alms from others and offer them to the spiritual master. However, one should not imitate the behavior of an advanced devotee or mahā-bhāgavata without being self-realized, for by such imitation one will eventually become degraded.
(Nectar of Instruction).

“As the Supersoul of the living entities, I sit in everyone's heart and observe everyone's activity in every stage and order of life. Regardless of which stage one is in, when I see that one is engaged seriously and sincerely in discharging the duties ordered by the spiritual master and is thus dedicating his life to the service of the spiritual master, that person becomes most dear to Me. As far as the life of brahmacarya is concerned, if one can continue the life of a brahmacārī under the direction of a spiritual master, that is extremely good; but if in brahmacārī life one feels sex impulses, he should take leave of his spiritual master, satisfying him according to the guru's desire. According to the Vedic system, a gift is offered to the spiritual master, which is called guru-dakṣiṇā. Then the disciple should take to householder life and accept a wife according to religious rites.”
(Krsna Book).

Śrī Nārada Muni is a naiṣṭhika-brahmacārī. There are four types of brahmacārīs. The first is called sāvitra, which refers to a brahmacārī who, after initiation and the sacred thread ceremony, must observe at least three days of celibacy. The next is called prājāpatya, which refers to a brahmacārī who strictly observes celibacy for at least one year after initiation. The next is called brāhma-brahmacārī, which refers to a brahmacārī who observes celibacy from the time of initiation up to the time of the completion of his study of the Vedic literature. The next stage is called naiṣṭhika, which refers to a brahmacārī who is celibate throughout his whole life. Out of these, the first three are upakurvāṇa, which means that thebrahmacārī can marry later, after the brahmacārī period is over. The naiṣṭhika-brahmacārī, however, is completely reluctant to have any sex life; therefore the Kumāras and Nārada are known as naiṣṭhika-brahmacārīs. Such brahmacārīs are called vīra-vrata because their vow of celibacy is as heroic as the vows of the kṣatriyas. The brahmacārī system of life is especially advantageous in that it increases the power of memory and determination. It is specifically mentioned in this connection that because Nārada was a naiṣṭhika-brahmacārī he could remember whatever he heard from his spiritual master and would never forget it. One who can remember everything perpetually is called a śruti-dhara. A śruti-dhara brahmacārī can repeat verbatim all that he has heard, without notes and without reference to books. The great sage Nārada has this qualification, and therefore, having taken instructions from Nārāyaṇa Ṛṣi, he is engaged in propagating the philosophy of devotional service all over the world. Because such great sages can remember everything, they are thoughtful, self-realized and completely fixed in the service of the Lord.
(Krsna Book).