The word "more well matured" refers to something that has been aged or developed to a desirable state. Some antonyms for this term could be "underdeveloped," "immature," "green," or "raw." These words suggest a lack of experience or refinement, and may be used to describe people, ideas, or products that are not quite ready for prime time. Other possible antonyms might include "untested," "unrefined," "undeveloped," or "novice." These terms all suggest a lack of experience or skill, and could be used to describe anything from a new business venture to an untrained athlete. Ultimately, the choice of antonym will depend on the subject being discussed, as well as the context in which it is being used.