Why does the prisoner tell Elie to say that he is eighteen and not fifteen?

The prisoner tells Elie to lie and say that he is eighteen, instead of giving his true age of fifteen, so that Elie will be counted as an adult and will be able to stay with his father.

This exchange takes place about a third of the way into the novel. Recall that Nightdoesn't have chapter divisions exactly, but rather sections; this conversion occurs in the section that begins with the phrase "The beloved...

The prisoner tells Elie to lie and say that he is eighteen, instead of giving his true age of fifteen, so that Elie will be counted as an adult and will be able to stay with his father.

This exchange takes place about a third of the way into the novel. Recall that Night doesn't have chapter divisions exactly, but rather sections; this conversion occurs in the section that begins with the phrase "The beloved objects."

Although the narration doesn't say so explicitly, it's clear that the prisoner with the "weary and warm" voice is anxious to help Elie and his father. He has already seen how the SS officer is asking for everyone's age, then directing adults to the left and children to the right, separating the children forever from their parents and, as it's strongly implied, sentencing the children to an immediate death. Although it's only a few years' difference, an eighteen-year-old is considered an adult while a fifteen-year-old is considered a child. And so with his kind insistence that Elie lie, the prisoner essentially saved Elie's life and extended the time Elie had left to spend with his father.