"Young elephants' food"
This means "the food of the young elephants" with the possessive form which does not exist in French.
All possessive constructions are identical in French: owned thing + de + definite article + owner.
"Des" here is not the plural of "un/une", but the contraction of de-les
Look at the singular forms: - la nourriture du jeune éléphant (= de-le) - la nourriture de la jeune éléphante