Change of body by the atomic individual soul is an accepted fact. Even the modern scientists who do not believe in the existence of the soul, but at the same time cannot explain the source of energy from the heart, have to accept continuous changes of body which appear from childhood to boyhood and from boyhood to youth and again from youth to old age. From old age, the change is transferred to another body.
(Bhagavad-Gita-----2:22-----purport).

In other words, Kṛṣṇa appears in this material world in His original eternal form, with two hands, holding a flute. He appears exactly in His eternal body, uncontaminated by this material world. Although He appears in the same transcendental body and is Lord of the universe, it still appears that He takes His birth like an ordinary living entity. And although His body does not deteriorate like a material body, it still appears that Lord Kṛṣṇa grows from childhood to boyhood and from boyhood to youth. But astonishingly enough He never ages beyond youth. At the time of the Battle of Kurukṣetra, He had many grandchildren at home; or, in other words, He had sufficiently aged by material calculations. Still He looked just like a young man twenty or twenty-five years old.
(Bhagavad-Gita-----4:6-----purport).

Everything that exists is a product of matter and spirit. Spirit is the basic field of creation, and matter is created by spirit. Spirit is not created at a certain stage of material development. Rather, this material world is manifested only on the basis of spiritual energy. This material body is developed because spirit is present within matter; a child grows gradually to boyhood and then to manhood because that superior energy, spirit soul, is present. Similarly, the entire cosmic manifestation of the gigantic universe is developed because of the presence of the Supersoul, Viṣṇu. Therefore spirit and matter, which combine to manifest this gigantic universal form, are originally two energies of the Lord, and consequently the Lord is the original cause of everything. A fragmental part and parcel of the Lord, namely the living entity, may be the cause of a big skyscraper, a big factory, or even a big city, but he cannot be the cause of a big universe. The cause of the big universe is the big soul, or the Supersoul. And Kṛṣṇa, the Supreme, is the cause of both the big and small souls. Therefore He is the original cause of all causes. This is confirmed in the Kaṭha Upaniṣad (2.2.13). Nityo nityānāṁ cetanaś cetanānām.
(Bhagavad-Gita-----7:6-----purport).

A living conditioned soul can thus understand that he is different from the body. It is described in the beginning—dehino 'smin—that the living entity is within the body and that the body is changing from childhood to boyhood and from boyhood to youth and from youth to old age, and the person who owns the body knows that the body is changing. The owner is distinctly kṣetra-jña. Sometimes we think, "I am happy," "I am a man," "I am a woman," "I am a dog," "I am a cat." These are the bodily designations of the knower. But the knower is different from the body. Although we may use many articles—our clothes, etc.—we know that we are different from the things used. Similarly, we also understand by a little contemplation that we are different from the body. I or you or anyone else who owns the body is called kṣetra-jña, the knower of the field of activities, and the body is called kṣetra, the field of activities itself.
(Bhagavad-Gita-----13:1-2-----purport).

So even in the material nature there is a chance of an independent choice by the living entity, and according to his choice the material energy offers him different varieties of material bodies. There are bodies, 1,100,000 worms and reptiles, 1,000,000 forms of birds, together there are 8,400,000 varieties of bodies in different planets of the universe, and the living entity is traveling by so many transmigrations according to different modes of enjoying spirit within himself. Even in one particular body the living entity changes from childhood to boyhood, from boyhood to youth, from youth to old age and from old age to another body created by his own action. The living entity creates his own body by his personal desires, and the external energy of the Lord supplies him the exact form by which he can enjoy his desires to the fullest extent.
(Srimad Bhagavatam-----2:9:2-----purport).

In His boyhood at Vṛndāvana, Lord Kṛṣṇa was notorious as a teasing friend in transcendental love to all the girls His age. His love for them was so intense that there is no comparison to that ecstasy, and the damsels of Vraja were so much attached to Him that their affection excelled that of the great demigods like Brahmā and Śiva. Lord Kṛṣṇa finally admitted His defeat before the transcendental affection of the gopīs and declared that He was unable to repay them for their unalloyed affection. Although the gopīs were seemingly anguished by the Lord's teasing behavior, when Kṛṣṇa would leave them they could not tolerate the separation and used to follow Him with their eyes and minds. They were so stunned by the situation that they could not finish their household duties.
(Srimad Bhagavatam-----3:2:14-----purport).

Nanda Mahārāja was due to receive Him as his child, and Yaśodāmayī was also to enjoy the childhood pastimes of the Lord, and therefore to fulfill everyone's desire, He was carried from Mathurā to Vṛndāvana just after His appearance in the prison house of Kaṁsa. He lived there for eleven years and completed all His fascinating pastimes of childhood,boyhood and adolescence with His elder brother, Lord Baladeva, His first expansion. Vasudeva's thought of protecting Kṛṣṇa from the wrath of Kaṁsa is part of a transcendental relationship. The Lord enjoys more when someone takes Him as his subordinate son who needs the protection of a father than He does when someone accepts Him as the Supreme Lord. He is the father of everyone, and He protects everyone, but when His devotee takes it for granted that the Lord is to be protected by the devotee's care, it is a transcendental joy for the Lord. Thus when Vasudeva, out of fear of Kaṁsa, carried Him to Vṛndāvana, the Lord enjoyed it; otherwise, He had no fear from Kaṁsa or anyone else.
(Srimad Bhagavatam-----3:2:26-----purport).

From birth to the end of five years of age is called childhood. After five years up to the end of the fifteenth year is called paugaṇḍa. At sixteen years of age, youth begins. The distresses of childhood are already explained, but when the child attains boyhood he is enrolled in a school which he does not like. He wants to play, but he is forced to go to school and study and take responsibility for passing examinations. Another kind of distress is that he wants to get some things with which to play, but circumstances may be such that he is not able to attain them, and he thus becomes aggrieved and feels pain. In one word, he is unhappy, even in his boyhood, just as he was unhappy in his childhood, what to speak of youth. Boys are apt to create so many artificial demands for playing, and when they do not attain satisfaction they become furious with anger, and the result is suffering.
(Srimad Bhagavatam-----3:31:28-----purport).

An important word in this verse is mukta-liṅgaḥ. Mukta means "liberated," and liṅga means "the subtle body." When a man dies, he quits the gross body, but the subtle body of mind, intelligence and ego carries him to a new body. While existing in the present body, the same subtle body carries him from one stage of life to another (for example, from childhood to boyhood) by mental development. The mental condition of a baby is different from that of a boy, the mental condition of a boy is different from that of a young man, and the mental condition of a young man is different from that of an old man. So at death the process of changing bodies takes place due to the subtle body; the mind, intelligence and ego carry the soul from one gross body to another. This is called transmigration of the soul. But there is another stage, when one becomes liberated even from the subtle body; at that time the living entity is competent and fully prepared to be transferred to the transcendental or spiritual world.
(Srimad Bhagavatam-----4:12:18-----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."
When a living entity transfers from one body to another, a process generally known as death, a sane man does not lament, for he knows that the living entity is not dead but is simply transferred from one body to another. The Queen should have been afraid of being alone in the forest with the body of her husband, but since she was a great wife of a great personality, she lamented for a while but immediately understood that she had many duties to perform. Thus instead of wasting her time in lamentation, she immediately prepared a fiery pyre on top of a hill and then placed the body of her husband on it to be burned.
(Srimad Bhagavatam-----4:23:21-----purport).