From the book Night.
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Sorry, I answered an older post of this question by mistake. I don’t know if the answer goes straight to you or not so I'll post it again.
He tells Elie and his father to lie about their age. Elie is told to say he is 18 (instead of 15) and Elie's father is told to say he is 40 (instead of 50). The Nazis felt therse ages were more valuable to them.