On the other hand, an adult brain has an advantage in that it already knows another language fluently, so complex words and concepts in the new language can be explained by way of translation, analogy, and deconstruction into the native language. An adult also has the advantage of structured learning and applying the most efficient language-learning strategy: systematic study of vocabulary and grammar, etc.

Adults do have a disadvantage as far as accents and fluency are concerned though. For that, you need total immersion (i.e. no relying on prior language as a crutch) and to completely reset oral posture rather than letting the oral posture of their native language influence speaking habits.