老成 describes a person who is mature, steady, and experienced beyond their years — it's a positive trait, often used to praise young people who act with composure and good judgment. The word combines 老 (old/experienced) and 成 (accomplished/mature), and is typically used as a stative verb (s.v.) in Chinese, functioning like an adjective in English. You can use it to describe someone's character or behavior, but not for objects or abstract concepts.