In the past, some students continued to reside with their guru without performing the samavartana. Even after his passing, they remained brahmacarins and remained so all their life.
Our dharma takes into account the natural urges of man. The general rule is that, on his return home from the gurukula, the student must marry and settle down. It is difficult to go against the natural urges. But going along with nature does not mean being swept away in the flow of urges. After all the goal of all our efforts is reaching the other shore-that is release from the worldly existence. The householder must lead a life of dharma with his wife. But later he must become a forest recluse first and then, renouncing everything, a sannyasin. This path to asceticism through stages is based on the fact that curbing the natural instincts is likely to be harmful. A person who decides in his youth to become a naisthika brahmacarin(lifelong student-bachelor)may later succumb to his natural passions. This would be an offence against the asrama code of conduct and therefore sinful. As a householder he is not guilty of any offence if he goes by his natural urges within the constraints of dharma.
There are exceptions to any rule. Those who have firmness and maturity of mind and strength of character obtained from the samskaras preformed in an earlier birth may become lifelong brahmacarins. We have the example of Samarta Ramadasa who lived more than 300 years ago. It was he who inspired Sivaji to uphold our dharma against the onslaught of Islam. Ramadasa, the naisthika brahmacarin, personified one aspect of Hanuman.
Sri Sankara Bhagavatpada was an incarnation of Paramesvara and his mission was re-establishment of the Vedic dharma. He went directly from the brahmacarya to the ascetic stage of life. His disciples too, with the exception of Suresvaracarya, did the same. Sankara gave initiation into sannyasa to Padmapada, Hastamalaka and Totaka. In the Sankara Matha also brahmacarins are initiated into sannyasa because, according to the rule, only such sannyasins can occupy the Pitha. All this points to the fact that everybody need not become a householder before donning the ascetic's garb. But it must be conceded that only a few will have the wisdom and mellowness necessary to skip two asramas (that of the householder and the forest recluse) to take to sannyasa. Naisthika brahmacarins do not have to perform the following samskaras: marriage, the five mahayajnas, the seven pakayajnas, the seven haviryajnas and the seven somayajnas. Their antah-karana must be sufficiently pure even without going through these rituals. So they are exceptional cases.
Dahana-kriya(cremation)is the last samskara according to the sastras. It is argued, on the basis of this, that they (the sastras)do not enjoin all, even the aged, to take to sannyasa. If everybody were to live through all the asramas (that of bachelor-student, householder, forest recluse and ascetic) there would truly be no question of the cremation rite for anybody. Are not sannyasins their attaining siddhi, instead of being consigned to the flames? If we believe that asceticism is only for the mature, and not even for the aged among the rest, the above argument cannot be said to be wrong.
The view that cremation rite applies only to those who die too young to become sannyasins is unfounded. Indeed not only those who die prematurely but also the old are to be cremated [if they do not become sannyasins]. So the inference is that sastras do accept aged people also not taking to sannyasa.
A person who has the light of knowledge in him and is free from passion must live in the forest giving up family responsibilities and performing only Vedic rites. He must leave his children and property behind and take only his wife with him to the forest. The wife, however, is not meant for carnal pleasure but is a partner in the conduct of rites involving the sacred fire-sacrifices, aupasana, etc. This is the meaning of vanaprastha. A person qualifies for this stage of life when he is mature enough to leave home and hearth, children and relatives. Later he gives up the Vedic karma itself and turns his mind exclusively to the quest of the Self. This is the time when he enters the sannyasasrama.
The man who has thus separated himself from his wife and given up Vedic works is initiated into sannyasa by his guru. He must constantly meditate on the Paramatman and experience the Truth as an inward reality. Also, he must have the realisation that, "That Truth am I, all else is false play. "Then he is by himself, beyond his body and mind, as the Ultimate Truth. This is moksa, liberation. Such a man will continue to dwell in his body until the fruits of his past karma are exhausted. But he will not be affected by such karma as a sannyasin who has inward realisation. From the point of view of the outside world he may still dwell in his body; but even in this state he is liberated. He is now a "jivanmukta". When the body perishes he becomes a "videhamukta"(liberated without the body). And he himself is now the unconditioned Ultimate Truth.
He who becomes a sannyasin without having lived as a householder and he who becomes a sannyasin after doing so, performing all the forty samskaras and acquiring all the eight Atmic qualities, becomes alike the Ultimate Truth.
What is the fate of the man who does not become an ascetic but who keeps performing, until his death, all the samskaras and cultivates the eight Atmic qualities? He is cremated on his death, is he not? After all, the majority of people belong to this category. What happens to such people after their death?
Sankara does not state that they will dissolve in Ultimate Reality. They do not have the intense urge, the burning desire, to grasp the Brahman, abandoning everything. If they have the all-consuming desire for the Truth, no force can hold them back from their quest. It is because they do not possess such a desire that they do not obtain non-dualistic release. However, they have faith in the sastras and perform works according to them and contribute to the well-being of the mankind and they are also thereby rendered pure inwardly. So, though they are not united with the Paramatman, they go to the presence of Isvara, Isvara who is the Paramatman with attributes(Saguna Brahman)and is behind the affairs of the world.
This is called "Hiranyagarbha-sthana" and it is the same as Brahmaloka. In this there is no inseparable dissolution in the Paramatman, but the man who attains it remains in bliss "experiencing" Isvara. Such a state is also to be described as moksa. There is nothing wanting, there is no sorrow, and there is the presence of the Lord. What more is wanted? This state is reached by those who perform all the samskaras even though they do not become ascetics.
But one day Isvara(the Saguna Brahman)will put a stop to the activities of all worlds and dissolve them in the great deluge(maha-pralaya). He will now become the Nirguna Brahman, the Paramatman without any attributes. At this time all those who reside by his side will unite with the Paramatman as the Paramatman, that is non-dualistic liberation.
In the great deluge all creatures-even those who have not performed any of the prescribed rituals, creatures like worms, reptiles and so on also -will merge with the Paramatman. Then what is special about the one who unites with the Supreme Being after having performed all the samskaras? When the Paramatman, as the Isvara with attributes, creates the worlds again those who do not perform the samskaras will be born again according to the karma of their past lives. Only those who have properly gone through the samskaras and been rendered pure will be inseparably united with the Brahman.
I have come far from the subject of upanayana. I had sought an answer to the following questions: "Can a person remain a brahmacarin all his life? Can a brahmacarin become a sannyasin without going through the stage of the householder? ''
For a general background, please see here