the level of educational development of an individual as determined by an achievement test and based on a comparison of the examinee's score with the average score of individuals of the same chronological age.
British Dictionary definitions for achievement age
achievement age
noun
1.
(psychol) the age at which a child should be able to perform a standardized test successfully Compare mental age
achievement age in Medicine
achievement age a·chieve·ment age (ə-chēv'mənt) n. The level of an individual's educational accomplishment as measured by standard achievement tests and expressed as the age in years of a child for whom the total score would be an average score.