The controversy over the optimal age for learning a second language is discussed, examining, from the perspective of Piagetian theory: (1) the argument which suggests that children have an advantage in language learning; and (2) the arguments which states that adults have an advantage in language learning. The first part provides an overview of the short- and long-term studies on child/adult differences that have led to the controversy, and points out some of the problems inherent in carrying out such studies. In part two, the major factors that have been suggested to account for age differences in second language learning are outlined. It is then argued that these factors fail to satisfactorily account for the differences between younger and older learners in both learning rate and ultimate attainment, thus biasing conclusions about optimal age; a resulting "disequilibrium model" of language learning is proposed. This model attempts, through application of the Piagetian concept of equilibration, to find a common ground on which the results of both long- and short-term studies can be examined collectively. It is concluded that determinations of optimal age in second language learning are incidental rather than substantive. A 65-item bibliography is included. (MSE)