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.
(Srimad Bhagavatam-----3:31:28-----purport).

Śrīmatī Rādhārāṇī first smears Her body with the paste of Kṛṣṇa's affection for Her. She then takes Her bath in the water of mercy. After passing the paugaṇḍa age (from five to ten years), Śrīmatī Rādhārāṇī first appears as mercy. The second bath is taken at noon in the water of tāruṇyāmṛta, or the nectar of youth. This is the actual expression of Her new youthfulness.
(Sri Caitanya Caritamrta-----2:8:167-----purport).

Kṛṣṇa's age is considered in three periods: from His appearance day to the end of His fifth year is called kaumāra, from the beginning of the sixth year up to the end of the tenth year is called paugaṇḍa, and from the eleventh to the end of the fifteenth year is called kaiśora. After the beginning of the sixteenth year, Kṛṣṇa is called a yauvana, or a youth, and this continues with no change.
As far as Kṛṣṇa's transcendental pastimes are concerned, they are mostly executed during the kaumāra, paugaṇḍa and kaiśora periods. His affectionate pastimes with His parents are executed during His kaumāra age. His friendship with the cowherd boys is exhibited during the paugaṇḍa period. And His friendship with the gopīs is exhibited during the age of kaiśora. Kṛṣṇa's pastimes at Vṛndāvana are finished by the end of His fifteenth year, and then He is transferred to Mathurā and Dvārakā, where all other pastimes are performed.
(Nectar of Devotion).

Learned scholars have divided Kṛṣṇa's age into three periods: the age up through five years is called kaumāra, the age from the sixth through the tenth year is called paugaṇḍa, and the age from the eleventh through fifteenth year is called kaiśora. While Kṛṣṇa is spending His days as a cowherd boy, He is in the kaumāra and paugaṇḍa ages. In the kaiśora age, when Kṛṣṇa appeared in Gokula, He acted as a cowherd boy, and then, when He was sixteen, He went to Mathurā to kill Kaṁsa.
(Nectar of Devotion).

Kṛṣṇa's paugaṇḍa age can be further divided into three periods—namely the beginning, middle and end. In the beginning of the paugaṇḍa age there is a very nice reddish luster on His lips, His abdomen is very thin, and on His neck are circles like those on a conchshell. Sometimes, some outside visitors would return to Vṛndāvana to see Kṛṣṇa and, upon seeing Him again, would exclaim, "My dear Mukunda, Your beauty is gradually increasing, just like the leaf on a banyan tree! My dear lotus-eyed one, Your neck is gradually manifesting circles like those of the conchshell. And in the shining moonlight Your teeth and cheeks are competing with the padmarāga jewels in their beautiful arrangement. I am sure that Your beautiful bodily development is now giving much pleasure to Your friends."
At this age Kṛṣṇa was garlanded with various kinds of flowers. He used to put on a silk dress, colored with various kinds of dye. Such beautiful decorations are considered cosmetics for Kṛṣṇa. Kṛṣṇa would wear this dress when He used to go into the forest to tend the cows. Sometimes He would wrestle there with His different friends, and sometimes they would dance all together in the forest. These are some of the specific activities of the paugaṇḍa age.
The cowherd friends of Kṛṣṇa were so happy in His company that they expressed their transcendental feelings within themselves thus: "Dear Kṛṣṇa, You are always busy tending the cows which are scattered all over beautiful Vṛndāvana. You have a beautiful garland, a small conchshell, a peacock feather on Your turban, yellow-colored silk cloth, decorations of karṇikāra flowers on Your ears and a mallikā flower garland on Your chest. Appearing so beautiful, when You pretend, just like an actor, to be fighting with us, You give us unlimited transcendental bliss."
(Nectar of Devotion).

When Kṛṣṇa is more grown up, in the middle age of paugaṇḍa, His nails become finely sharp, and His chubby cheeks become lustrous and round. On the two sides of His waist above His belt there are three distinct lines of folded skin, called trivalī.
The cowherd boyfriends of Kṛṣṇa felt very proud of their association with Him. At that time the tip of His nose defeated the beauty of sesame flowers, the luster of His cheeks defeated the glow of pearls, and the two sides of His body were exquisitely beautiful. In this age Kṛṣṇa wore a silk dress that glittered like lightning, His head was decorated with a silk turban covered with gold lace, and in His hand He carried a stick about fifty-six inches long.* Seeing this exquisitely beautiful dress of Kṛṣṇa, one devotee addressed his friend in this manner: "My dear friend, just look at Kṛṣṇa! See how He is carrying in His hand a stick which is bound up and down with golden rings, how His turban with golden lace is showing such a beautiful luster, and how His dress is giving His friends the highest transcendental pleasure!"
(Nectar of Devotion).

At the end of Kṛṣṇa's paugaṇḍa age, Kṛṣṇa's hair sometimes hangs down to His hips, and sometimes it becomes scattered. In this age His two shoulders become higher and broader, and His face is always decorated with marks of tilaka. When His beautiful hair scatters over His shoulders, it appears to be a goddess of fortune embracing Him, and this embracing is highly relished by His friends. Subala once addressed Him in this way: "My dear Keśava, Your round turban, the lotus flower in Your hand, the vertical marks of tilaka on Your forehead, Your kuṅkuma-flavored musk and all of Your beautiful bodily features are defeating me today, although I am usually stronger than You or any of our friends. Since this is so, I do not know how these features of Your body can fail to defeat the pride of all the young girls of Vṛndāvana. When I am so defeated by this beauty, what chance is there for those who are naturally very simple and flexible?"
At this age Kṛṣṇa took pleasure in whispering into the ears of His friends, and the subject of His talks was the beauty of the gopīs, who were just tarrying before them. Subala once addressed Kṛṣṇa thus: "My dear Kṛṣṇa, You are very cunning. You can understand the thoughts of others; therefore I am whispering within Your ear that all of these five gopīs, who are most beautiful, have been attracted by Your dress. And I believe that Cupid has entrusted them with the responsibility of conquering You." In other words, the beauty of the gopīs was capable of conquering Kṛṣṇa, although Kṛṣṇa is the conqueror of all universes.
(Nectar of Devotion).

When Kṛṣṇa, the beloved son of Nanda Mahārāja, steps into His kaiśora age, although He becomes more beautiful, His parents still consider Him to be in thepaugaṇḍa age—even though He is between the ages of ten and fifteen. When Kṛṣṇa is in His paugaṇḍa age, some of His servants also accept Him as being in the kaiśora age. When Kṛṣṇa performs His childish pastimes, His general practice is to break the milk and yogurt pots, throw the yogurt in the courtyard and steal the cream from the milk. Sometimes He breaks the churning rod, and sometimes He throws butter on the fire. In this way, He increases the transcendental pleasure of His mother, Yaśodā.
(Nectar of Devotion).

The killing of Aghāsura took place when Kṛṣṇa and all His boyfriends were under five years old. Children under five years old are called kaumāra, from five years up to the tenth year they are called paugaṇḍa, and from the tenth year up to the fifteenth year they are called kaiśora. After the fifteenth year, boys are called youths. For one year there was no discussion of the incident of the Aghāsura demon in the village of Vraja. But when the boys attained their sixth year, they informed their parents of the incident with great wonder.
(Krsna Book).

King Parīkṣit was a bit astonished to understand that the killing of the Aghāsura demon was not discussed for one year, until after the boys attained the paugaṇḍa age. Mahārāja Parīkṣit was very inquisitive to learn about this, for he was sure that such an incident was due to the working of Kṛṣṇa's different energies.
(Krsna Book).

In this way Śrī Kṛṣṇa, along with His elder brother Balarāma, passed the childhood age known as kaumāra and stepped into the age of paugaṇḍa, from the sixth year up to the tenth. At that time, all the cowherd men conferred and agreed to give those boys who had passed their fifth year charge of the cows in the pasturing ground. Given charge of the cows, Kṛṣṇa and Balarāma traversed Vṛndāvana, purifying the land with Their lotus footprints.
(Krsna Book).