why did elie lie about his age and occupation when he was question by the officer

He lied because he didn't want to be separated from his father. Another prisoner had advised both Eli and his father to lie.

Since, he was so young, if Elie had been truthful about his age, he very likely would have been sent to the gas chamber during the initial selection. Both his father & he lied about their ages for that very reason: Elie lied about being 18(older) & his father lied about being 40 (younger).