Growing into youth one's senses sometimes become disturbed and celibacy becomes difficult. The Kumaras therefore purposefully remained children because in a child's life the senses are never disturbed by sex
SB Canto 4
Mahārāja Pṛthu offered his welcome to the four Kumāras, addressing them as the best of the brāhmaṇas. He welcomed them, saying: From the beginning of your birth you strictly observed the vows of celibacy, and although you are experienced in the path of liberation, you are keeping yourselves just like small children.
The specific importance of the Kumāras is that they were brahmacārīs, living the life of celibacy from birth. They kept themselves as small children about four or five years old because by growing into youth one's senses sometimes become disturbed and celibacy becomes difficult. The Kumāras therefore purposefully remained children because in a child's life the senses are never disturbed by sex. This is the significance of the life of the Kumāras, and as such Mahārāja Pṛthu addressed them as the best of the brāhmaṇas. Not only were the Kumāras born of the best brāhmaṇa (Lord Brahmā), but they are addressed herein as dvija-śreṣṭhāḥ ("the best of the brāhmaṇas") on account of their being Vaiṣṇavas also. As we have already explained, they have their sampradāya (disciplic succession), and even to date the sampradāya is being maintained and is known as the Nimbārka-sampradāya. Out of the four sampradāyas of the Vaiṣṇava ācāryas, the Nimbārka-sampradāya is one. Mahārāja Pṛthu specifically appreciated the position of the Kumāras because they maintained the brahmacarya vow from the very beginning of their birth. Mahārāja Pṛthu, however, expressed his great appreciation of Vaiṣṇavism by addressing the Kumāras as vaiṣṇava-śreṣṭhāḥ. In other words, everyone should offer respect to a Vaiṣṇava without considering his source of birth. Vaiṣṇave jāti-buddhiḥ. No one should consider a Vaiṣṇava in terms of birth. The Vaiṣṇava is always the best of the brāhmaṇas, and as such one should offer all respects to a Vaiṣṇava, not only as a brāhmaṇa but as the best of the brāhmaṇas.