In the Prologue to Oedipus Rex, the priest mentions boys, young men, and multitudes. Whom does he represent?
Sophocles' Oedipus the King opens at the palace of the Theban king Oedipus. A terrible plague has struck the town and the palace is beseiged by various groups of Thebans, who are worried about the plight that grips their city. Oedipus is the first to speak and he address the priest as an "old man". Oedipus also says that this "old man" appears "to be the one / who ought to speak for those assembled here." Thus, it seems that this "old man" represents all of the people assembed at Oedipus' palace.
The "old man" identifies himself further. He says, "I’m priest of Zeus." Thus, not only does he speak for those gathered at Oedipus' house, but he also is a representative of the god Zeus.
This old priest also notes that he and the others are "here as suppliants / all begging you to find some help for us."
So, in sum, this man is an aged priest of Zeus who speaks on behalf of all of the Theban people suffering under the burden of the plague.