When questioned by the s. s. officer, why did elie lie about his age and occupation?

because if they were a certain age they would not be sent straight to the gas chamber. If they were eighteen and had an occupation they would be sent to work instead of sent to die

At that time, the nazi Germany still in the middle of their battle to conquer both the eastern and western front in European world war II.

Because of this, they need a large quantity of factory workers to make their weapons that mostly come from the jewish people in productive age.