Language learning aptitude does not refer to whether or not an individual can or cannot learn a foreign language. It is assumed that virtually everybody can learn a language given adequate opportunity.
According to John B. Carroll and Stanley Sapon, the authors of the Modern Language Aptitude Test, language learning aptitude does refer to the “prediction of how well, relative to other individuals, an individual can learn a foreign language in a given amount of time and under given conditions."
As with many measures of aptitude, language learning aptitude is thought to be relatively stable throughout an individual’s lifetime.