Bhakti-rasāmṛta-sindhu .1.309
Blended reading (Sanskrit + English inline)
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.
This is the general definition. Kṛṣṇa however should be understood to be special. Thus, there are verses such as following: kālenālpena rājarṣe rāmaḥ kṛṣṇaś ca gokule aghṛṣṭa-jānubhiḥ padbhir vicakramatur añjasā O King Parīkṣit, within a very short time both Rāma and Kṛṣṇa began to walk very easily in Gokula on Their legs, by Their own strength, without the need to crawl. SB 10.8.26
My note
Source: vedabase.io