I will certainly tell you, Socrates, what my own experience of it is. I and a few other people of my own age are in the habit of frequently meeting together, true to the old proverb. On these occasions, most of us give way to lamentations, and regret the pleasures of youth, and call up the memory of amours and drinking parties and banquets and similar proceedings. - The Republic of Plato
What's the meaning of true to the old proverb? I couldn't find the old proverb.
This phrase indicates that a proverb is being fulfilled, although not directly cited. (You didn't miss it; it does not appear in this text.) The idea is that the reader will recognize the situation and recall the proverb. The situation is "true to" the proverb, meaning that it shows the truth of the proverb.
If the context were modern English, the old proverb would probably be, "Birds of a feather flock together," meaning that people tend to associate with those who are similar to themselves. Indeed, some translators of The Republic allude to this proverb in the quoted passage.
In this case, we need a proverb that was common in Athens around the time of Socrates: "ἧλιξ ἥλικα τέρπει". It translates roughly as, "two of the same age delight each other", and it appears directly in another of Plato's dialogues, The Phaedrus.
