幼小 is a literary, attributive adjective used to describe living things (children, animals, plants) that are young and small — physically in an early, tender stage of development. Unlike the English gloss 'immature', 幼小 does not describe childish or irresponsible behavior (use 幼稚 for that). It commonly appears in set phrases like 幼小的心灵 ('a tender/young heart/mind'). You cannot use it predicatively (e.g., '这个孩子很幼小' sounds off); it always precedes a noun.