54
Śrīmad-Bhāgavatam
[Canto 6, Ch. 7]
na garhayanti hy artheṣu 
    yaviṣṭhāṅghry-abhivādanam 
    chandobhyo 'nyatra na brahman 
    vayo jyaiṣṭhyasya kāraṇam
na—not; garhayanti—forbid; hi—indeed; artheṣu—in acquiring interests; yaviṣṭha-aṅghri—at the lotus feet of a junior; abhivādanam—offering obeisances; chandobhyaḥ—the Vedic mantras; anyatra—apart from; na—not; brahman—O brāhmaṇa; vayaḥ—age; jyaiṣṭhyasya—of seniority; kāraṇam—the cause.
TRANSLATION
The demigods continued: Do not fear criticism for being younger than us. Such etiquette does not apply in regard to Vedic mantras. Except in relationship to Vedic mantras, seniority is determined by age, but one may offer respectful obeisances even to a younger person who is advanced in chanting Vedic mantras. Therefore although you are junior in relationship to us, you may become our priest without hesitation.
PURPORT
It is said,  vṛddhatvaṁ vayasā vinā : one may be senior without being advanced in age. Even if one is not old, one gains seniority if he is senior in knowledge. Viśvarūpa was junior in relationship to the demigods because he was their nephew, but the demigods wanted to accept him as their priest, and therefore he would have to accept obeisances from them. The demigods explained that this should not be a cause for hesitation; he could become their priest because he was advanced in Vedic knowledge. Similarly,  Cāṇakya Paṇḍita  advises,  nīcād apy uttamaṁ jñānam : one may accept education from a member of a lower social order. The  brāhmaṇas , the members of the most elevated  varṇa , are teachers, but a person in a lower family, such as a family of  kṣatriyas ,  vaiśyas  or even  śūdras , may be accepted as a teacher if he has knowledge. Śrī Caitanya Mahāprabhu approved of this when He expressed this opinion before Rāmānanda Rāya (Cc.  Madhya  8.128):
kibā vipra, kibā nyāsī, śūdra kene naya 
    yei kṛṣṇa-tattva-vettā, sei 'guru' haya