When He (Krsna) reaches the age of pre-youth, kaisora, He does not grow any older. He simply remains in His kaisora age. He is therefore described in the Brahma-samhita (BS 5.38) as nava-yauvana

Expressions researched:
"When He reaches the age of pre-youth, kaiśora, He does not grow any older. He simply remains in His kaiśora age. He is therefore described in the Brahma-saṁhitā (BS 5.38) as nava-yauvana"

Sri Caitanya-caritamrta

CC Madhya-lila

The comparison made here is very interesting. Kṛṣṇa does not grow like an ordinary human being, even though He exhibits His pastimes of childhood, boyhood and pre-youth. When He reaches the age of pre-youth, kaiśora, He does not grow any older. He simply remains in His kaiśora age. He is therefore described in the Brahma-saṁhitā (BS 5.38) as nava-yauvana.

CC Madhya 20.384, Translation and Purport

“Lord Kṛṣṇa exhibits His pastimes of childhood, boyhood and pre-youth. When He reaches pre-youth, He continues to exist eternally to perform His rāsa dance and other pastimes.

The comparison made here is very interesting. Kṛṣṇa does not grow like an ordinary human being, even though He exhibits His pastimes of childhood, boyhood and pre-youth. When He reaches the age of pre-youth, kaiśora, He does not grow any older. He simply remains in His kaiśora age. He is therefore described in the Brahma-saṁhitā (BS 5.38) as nava-yauvana:

advaitam acyutam anādim ananta-rūpam

ādyaṁ purāṇa-puruṣaṁ nava-yauvanaṁ ca

vedeṣu durlabham adurlabham ātma-bhaktau

govindam ādi-puruṣaṁ tam ahaṁ bhajāmi

This nava-yauvana, or pre-youth, is the eternal transcendental form of Kṛṣṇa. Kṛṣṇa never grows older than nava-yauvana.