At the fag end of life, when the invalidity of old age attacks a man, his body becomes useless for all purposes. Therefore Vedic training dictates that when a man is in his boyhood he should be trained in the process of brahmacarya; that is, he should be completely engaged in the service of the Lord and should not in any way associate with women. When the boy becomes a young man, he marries between the ages of twenty and twenty-five. When he is married at the right age, he can immediately beget strong, healthy sons. Now female descendants are increasing because young men are very weak sexually. A male child will be born if the husband is sexually stronger than the wife, but if the female is stronger, a female child will be born. Thus it is essential to practice the system of brahmacarya if one wishes to beget a male child when one is married.
(Srimad Bhagavatam-----4:28:3-----purport).

"As the embodied soul continually passes, in this body, from boyhood to youth to old age, the soul similarly passes into another body at death. The self-realized soul is not bewildered by such a change." We are not the body; we are spiritual beings trapped in the body. Our real interest lies in understanding this simple fact. Then we can make further spiritual progress. Otherwise, if we remain in the bodily conception of life, our miserable material existence will continue forever. Political adjustments, social welfare work, medical assistance and the other programs we have manufactured for peace and happiness will never endure. We shall have to undergo the sufferings of material life one after another. Therefore material life is said to be duḥkhālayam aśāśvatam (B.G.8:15); it is a reservoir of miserable conditions.
(Srimad Bhagavatam-----6:15:3-----purport).

The living entity is certainly eternal, but because he is covered by a temporary dress, the body, his eternity is not observed. Dehino 'smin yathā dehe kaumāraṁ yauvanaṁ jarā: (B.G.2:13) "The embodied soul continually passes, in this body, from boyhood to youth to old age." Thus the bodily dress is impermanent. The living entity, however, is permanent. As an animal is transferred from one owner to another, the living entity who was the son of Citraketu lived as his son for some time, but as soon as he was transferred to another body, the affectionate relationship was broken. As stated in the example given in the previous verse, when one has a commodity in his hands he considers it his, but as soon as it is transferred it becomes someone else's commodity. Then one no longer has a relationship with it; he has no affection for it, nor does he lament for it.
(Srimad Bhagavatam-----6:16:17-----purport).

The purport is that as long as we are in this human form of body, our duty is to understand the soul within the body. The body is not the self; we are different from the body, and therefore there is no question of friends, enemies or responsibilities in terms of the bodily conception of life. One should not be very anxious about the body's changing from childhood to boyhood, from boyhood to old age and then to apparent annihilation. Rather, one should be very seriously concerned about the soul within the body and how to release the soul from the material clutches. The living entity within the body is never annihilated; therefore one should surely know that whether one has many friends or many enemies, his friends cannot help him, and his enemies cannot do him any harm. One should know that he is a spirit soul (ahaṁ brahmāsmi) and that the constitutional position of the soul is unaffected by the changes of the body. In all circumstances, everyone, as a spirit soul, must be a devotee of Lord Viṣṇu and should not be concerned with bodily relationships, whether with friends or with enemies. One should know that neither we ourselves nor our enemies in the bodily conception of life are ever killed.
(Srimad Bhagavatam-----7:2:60-----purport).

Without Kṛṣṇa consciousness, one wastes twenty years in childhood and boyhood and another twenty years in old age, when one cannot perform any material activities and is full of anxiety about what is to be done by his sons and grandsons and how one's estate should be protected. Half of these years are spent in sleep. Furthermore, one wastes another thirty years sleeping at night during the rest of his life. Thus seventy out of one hundred years are wasted by a person who does not know the aim of life and how to utilize this human form.
(Srimad Bhagavatam-----7:6:7-----purport).

This is the account of one hundred years of life. Although in this age a lifetime of one hundred years is generally not possible, even if one has one hundred years, the calculation is that fifty years are wasted in sleeping, twenty years in childhood and boyhood, and twenty years in invalidity (jarā-vyādhi). This leaves only a few more years, but because of too much attachment to household life, those years are also spent with no purpose, without God consciousness. Therefore, one should be trained to be a perfect brahmacārī in the beginning of life and then to be perfect in sense control, following the regulative principles, if one becomes a householder. From household life one is ordered to accept vānaprastha life and go to the forest and then accept sannyāsa. That is the perfection of life. From the very beginning of life, those who are ajitendriya, who cannot control their senses, are educated only for sense gratification, as we have seen in the Western countries. Thus the entire duration of a life of even one hundred years is wasted and misused, and at the time of death one transmigrates to another body, which may not be human. At the end of one hundred years, one who has not acted as a human being in a life of tapasya (austerity and penance) must certainly be embodied again in a body like those of cats, dogs and hogs. Therefore this life of lusty desires and sense gratification is extremely risky.
(Srimad Bhagavatam-----7:6:8-----purport).

"As the embodied soul continually passes, in this body, from boyhood to youth to old age, the soul similarly passes into another body at death. The self-realized soul is not bewildered by such a change." (B.G.2:13) Thus the first instruction is that one should understand that the soul is within the body and is transmigrating from one body to another. This is the beginning of spiritual knowledge. Any person who is not expert in understanding this science or is unwilling to understand it remains in the bodily conception of life, or the animalistic conception of life, as confirmed in Śrīmad-Bhāgavatam (yasyātma-buddhiḥ kuṇape tri-dhātuke. .. sa eva go-kharaḥ (S.B.10:84:13). Every member of human society should clearly understand the instructions of Bhagavad-gītā, for only in this way can one be spiritually elevated and automatically give up the false, illusory knowledge by which one thinks, "I am this body, and everything belonging to this body is mine (ahaṁ mameti SB 5.5.8)." This doggish conception should be rejected immediately. One should be prepared to understand the spirit soul and the supreme spirit, God, who are eternally related. Thus one may return home, back to Godhead, having solved all the problems of life.
(Srimad Bhagavatam-----7:7:21-----purport).

Of course, a devotee should try to perfect his Kṛṣṇa consciousness in this life, for simply by understanding Kṛṣṇa and His instructions, after giving up this body one can return home, back to Godhead. Even if there is some falldown, practice of Kṛṣṇa consciousness never goes in vain. For example, Ajāmila, in his boyhood, practiced chanting the name of Nārāyaṇa under the direction of his father, but later, in his youth, he fell down and became a drunkard, woman-hunter, rogue and thief. Nonetheless, because of chanting the name of Nārāyaṇa for the purpose of calling his son, whom he had named Nārāyaṇa, he became advanced, even though he was involved in sinful activities. Therefore, we should not forget the chanting of the Hare Kṛṣṇa mantra under any circumstances. It will help us in the greatest danger, as we find in the life of Gajendra.
(Srimad Bhagavatam-----8:3:1-----purport).

The Personality of Godhead manifests Himself in six different features: (1) prābhava, (2) vaibhava, (3) empowered incarnations, (4) partial incarnations, (5) childhood and (6) boyhood. The Personality of Godhead Śrī Kṛṣṇa, whose permanent feature is adolescence, enjoys His transcendental proclivities by performing pastimes in these six forms. In these six features there are unlimited divisions of the Personality of Godhead's forms. The jīvas, or living beings, are differentiated parts and parcels of the Lord. They are all diversities of the one without a second, the Supreme Personality of Godhead.
(Sri Caitanya Caritamrta-----1:2:100-----purport).

Lord Kṛṣṇa remains in this universe for only 125 years, but all the pastimes of that period are exhibited in each and every universe. These pastimes include His appearance, the activities of His boyhood and youth, and His later pastimes, including those at Dvārakā. Since all these pastimes are present in one or another of the myriad universes at any given time, they are called eternal. Just as the sun is eternally existing, although we see it rise and set, appear and disappear, according to our position on the earth, so Kṛṣṇa's pastimes are eternally going on, although we can see them in this particular universe only at certain intervals. As stated earlier, Kṛṣṇa's abode is the supreme planet, known as Goloka Vṛndāvana, and by His will this Goloka Vṛndāvana is manifested in this universe and in other universes as well. Like Kṛṣṇa's name, fame and everything else directly connected to Him, Goloka Vṛndāvana is absolute and is therefore equal to Him.
(Teachings of Lord Caitanya).