Antonyms for the phrase "acts one's age" can include "immature," "childish," "adolescent," "juvenile," and "irresponsible." When someone is not acting their age, they may behave in a way that is inappropriate or insensitive to their surroundings. This can manifest as acting impulsively or recklessly, neglecting responsibilities or obligations, or prioritizing fun over more serious matters. Conversely, acting one's age entails displaying maturity, responsibility, and a balanced perspective that responds appropriately to life's challenges. It reflects a sense of wisdom gained through experience and an ability to adapt and adjust to different social situations. Ultimately, acting one's age is a sign of emotional intelligence and a strong sense of self-awareness.