Antonyms for the phrase "young one" include "elder," "senior," "adult," and "experienced." These words represent the opposite end of the age spectrum, highlighting the contrast between youth and maturity. "Elder" or "senior" specifically refers to someone who is older in age, while "adult" implies someone who has reached legal age and can take responsibility for their actions. "Experienced" refers to a person who has gained knowledge over time and can apply this to different situations. Collectively, these antonyms emphasize the importance of gaining wisdom and experience as people grow older, and the value of respecting one's elders.