what do socrates' questions to cephalus about old age,death,wealth have to do with the discussion about justice that follows

In the strictest Platonic sense, it might not be wrong to say that this has nothing to do at all with justice as a reality. This is because, for Plato, justice is immune to the vicissitudes of history (which include aging, death, money, etc.). But, in the context of the actual narrative, it is important because Socrates is able to show that justice is not conditioned by these things.