When most authors refer to “ultimate attainment” of an L2, they refer to the ability to attain native-like proficiency in an L2.
In many past studies, it has been shown that children, as compared to adults, are more likely to attain native-like proficiency at a second language, especially in areas such as grammar, and the production and perception of speech (Finn, Lee, Kraus, & Kam, 2014).