The antonyms for the word "most newborn" are tricky to pinpoint since it is an unusual phrase. However, one possible antonym could be "oldest" or "most elderly," referring to someone who has reached an advanced age. Another could be "fully grown," indicating that the person has already reached maturity. Other antonyms could be "experienced," "veteran," or "seasoned," implying that the person has gained years of knowledge and expertise in a particular field or area. Whatever the antonym may be, it is essential to choose the correct word in the right context to avoid confusion or misunderstanding.