One must leave his family life and enter the forest after the age of fifty. This is an authoritative statement of the Vedas, based on the division of social life into four departments of activity - brahmacarya, grhastha, vanaprastha and sannyasa
SB Canto 3
The Personality of Godhead Kapila said: Whatever I speak, whether directly or in the scriptures, is authoritative in all respects for the people of the world. O Muni, because I told you before that I would become your son, I have descended to fulfill this truth.
Kardama Muni was to leave his family life to completely engage in the service of the Lord. But since he knew that the Lord Himself, as Kapila, had taken birth in his home as his own son, why was he preparing to leave home to search out self-realization or God realization? God Himself was present in his home—why should he leave home? Such a question may certainly arise. But here it is said that whatever is spoken in the Vedas and whatever is practiced in accordance with the injunctions of the Vedas is to be accepted as authoritative in society. Vedic authority says that a householder must leave home after his fiftieth year. Pañcāśordhvaṁ vanaṁ vrajet: one must leave his family life and enter the forest after the age of fifty. This is an authoritative statement of the Vedas, based on the division of social life into four departments of activity—brahmacarya, gṛhastha, vānaprastha and sannyāsa.
Kardama Muni practiced yoga very rigidly as a brahmacārī before his marriage, and he became so powerful and attained so much mystic power that his father, Brahmā, ordered him to marry and beget children as a householder. Kardama did that also; he begot nine good daughters and one son, Kapila Muni, and thus his householder duty was also performed nicely, and now his duty was to leave. Even though he had the Supreme Personality of Godhead as his son, he had to respect the authority of the Vedas. This is a very important lesson. Even if one has God in his home as his son, one should still follow the Vedic injunctions. It is stated, mahājano yena gataḥ sa panthāḥ: (CC Madhya 17.186) one should traverse the path which is followed by great personalities.
Kardama Muni's example is very instructive, for in spite of having the Supreme Personality of Godhead as his son, he left home just to obey the authority of the Vedic injunction. Kardama Muni states here the main purpose of his leaving home: while traveling all over the world as a mendicant, he would always remember the Supreme Personality of Godhead within his heart and thereby be freed from all the anxieties of material existence. In this age of Kali-yuga sannyāsa is prohibited because persons in this age are all śūdras and cannot follow the rules and regulations of sannyāsa life. It is very commonly found that so-called sannyāsīs are addicted to nonsense—even to having private relationships with women. This is the abominable situation in this age. Although they dress themselves as sannyāsīs, they still cannot free themselves from the four principles of sinful life, namely illicit sex life, meat-eating, intoxication and gambling. Since they are not freed from these four principles, they are cheating the public by posing as svāmīs.
In Kali-yuga the injunction is that no one should accept sannyāsa. Of course, those who actually follow the rules and regulations must take sannyāsa. Generally, however, people are unable to accept sannyāsa life, and therefore Caitanya Mahāprabhu stressed, kalau nāsty eva nāsty eva na-sty eva gatir anyathā. In this age there is no other alternative, no other alternative, no other alternative than to chant the holy name of the Lord: Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare. The main purpose of sannyāsa life is to be in constant companionship with the Supreme Lord, either by thinking of Him within the heart or hearing of Him through aural reception. In this age, hearing is more important than thinking because one's thinking may be disturbed by mental agitation, but if one concentrates on hearing, he will be forced to associate with the sound vibration of Kṛṣṇa. Kṛṣṇa and the sound vibration "Kṛṣṇa" are nondifferent, so if one loudly vibrates Hare Kṛṣṇa, he will be able to think of Kṛṣṇa immediately. This process of chanting is the best process of self-realization in this age; therefore Lord Caitanya preached it so nicely for the benefit of all humanity.