The phrase "in one's prime" refers to the peak years of one's physical or mental abilities, usually between the ages of 25 to 50. It suggests that a person is at the height of their power, strength, and energy. Antonyms of "in one's prime" would therefore be words that describe a decline in health, energy, or ability. Some antonyms could include "past one's prime," "past their heyday," "over the hill," or "past their peak." These words describe a person who is no longer in their prime and has passed their most vital years. It could also refer to someone who has lost their energy, strength, or power.