Near the beginning of the Politics Aristotle says that the young should not study politics, for they have not yet acquired the experience requisite for such a science.  Perhaps a similar concern, free of prohibition, offers Plato a reason to introduce the character of Cephalus at the beginning of the Republic.

Cephalus, as it turns out, is a very old man.(1)  Socrates tells us that he is very eager to speak to the aged, because all have to travel down the path of old age, and he wants to know “what kind of road it is, rough and difficult, or easy and passable.”(2)  Plato uses Cephalus, as seems plain from his hasty exit, for a characteristically dramatic and philosophical purpose.  Cephalus himself elaborates on the topic of old age, eagerly opining to a rare audience, Socrates, that although the carousing long nights of youth are no longer possible, gone also is the despotism of sexual desire.  More importantly, he mentions that though wealth is a palliative in old age, character is more important.(3)  More somberly, Cephalus continues on to admit that of late he has been haunted by the fear of his life after death— the hand of death is beginning to point at him.  “The tales told concerning those in Hades, how it is necessary that those who have been unjust must pay the penalty there, although entirely laughable up to this point, at that point [of encroaching mortality] they harrow his soul that they may be true.” (4)

Cephalus emphasizes however, that just deeds, stored and accumulated throughout a life, are a sweet hope to the man in old age that he will escape such horrors.  It is from Cephalus then, by the exortation of old age, the idea that a complete life must be lived justly in order to reap the easy conscience of old age, and that character, informed by the requirements of justice may allow a soul to pass into the blessed afterworld, that the great themes of justice and morality are introduced into the dialogue.  Far from being an abstract treatise, Plato foresees for us a path that all must take, that of a burdensome or light old age, and gives us no determination of the fate of Cephalus, but bids us examine our own.

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Translations mine.

1. 328b9 μάλα πρεσβύτης
2. 328e3-4 ποία τίς ἐστιν, τραχεῖα καὶ χαλεπή,
   ἢ ῥᾳδία καὶ εὔπορος.
3. 329d3 ὁ τρόπος
4. 330d7-e2 οἵ τε γὰρ λεγόμενοι μῦθοι περὶ
   τῶν ἐν Ἅιδου, ὡς τὸν ἐνθάδε ἀδικήσαντα δεῖ ἐκεῖ διδόναι δίκην, καταγελώμενοι τέως, τότε δὴ στρέφουσιν αὐτοῦ τὴν ψυχὴν μὴ ἀληθεῖς ὦσιν·