Generally, among ISKCON devotees the position of seniority is determined by diksa, but other factors may take precedence depending on the circumstances. If a younger devotee has a position of authority over an older devotee, he must instruct the older devotee but still he must do so with due respect regarding the older devotee's seniority by initiation. Age is another factor. An older person who is new to Krishna consciousness must still be offered due respect because of his or her age even if a younger devotee is senior by initiation.
But above and beyond all of these considerations is the overriding element of spiritual advancement. If a devotee is clearly far superior by spiritual advancement, he is given all respect. Sukadeva Goswami is the classic example. When he arrived at the bank of the Ganges where Maharaja Pariksit was seated among all the sages from all over the universe, the sages were trying to determine who was the most qualified to speak to Maharaja Pariksit. Sukadeva was only sixteen-years-old. He was probably the most junior person present by all other considerations. But all the sages unanimously chose him, even above his father, Vyasadeva, because of his superior spiritual realisation.
Among materialists, an atmosphere of envy predominates: superiors are envied and dragged down, juniors are trodden on and exploited, and peers are competed with. See Srimad-Bhagavatam 4.8.34.