The term "attain majority" refers to the point at which a person reaches legal adulthood, typically 18 years old in most countries. Antonyms for this phrase could include "remain a minor," "fail to reach adulthood," or "stay under legal age." Other possible antonyms might be "lose majority," indicating a loss of legal adulthood, or "revert to minority," indicating a return to a state of being a minor. However, all of these terms imply a negative outcome or inability to achieve the legal status of adulthood, making "attain majority" a more positive and aspirational phrase.