SB 6.7.33

na garhayanti hy artheṣu
 yaviṣṭhāṅghry-abhivādanam
chandobhyo ’nyatra na brahman
 vayo jyaiṣṭhyasya kāraṇam
Synonyms: 
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

It does not matter whether one is a brāhmaṇa, śūdra, gṛhastha or sannyāsī. These are all material designations. A spiritually advanced person has nothing to do with such designations. Therefore, if one is advanced in the science of Kṛṣṇa consciousness, regardless of his position in human society, he may become a spiritual master.