His way of thinking is still immature.
Unripe fruit cannot be harvested.
Similar meaning of immaturity or inexperience, but 未熟 is more common in everyday language and often refers to skill or experience, while 未成熟 can sound more formal or biological.
maturity; ripeness
immature ovum; immature egg
Compound of 未 (mi, 'not yet') and 成熟 (seijuku, 'maturity'). The prefix 未- negates the state of being mature.