To expand on Douglas_Knight's answer, if it seems counter intuitive that is because children spend basically all their waking hours in a learning environment for nearly two decades of their life. Most of that time is spent learning or using in some way one or more languages.
The typical adult method for learning a second langauge, on the other hand, is to spend an hour or two a week in a classroom or with a tutor. No wonder it doesn't work as well.
To compare apples-to-apples, consider for example the Monterey Naval Postgraduate School which trains American soldiers and intelligence officers. Using a full-immersion, 24/7 learning environment they are able to take adult learners from zero to near practical fluency in months to years (depending on the difficulty of the language). Similar results are reported with Peace Core volunteers, for example, at least those which find themselves in a fully non-English environment.
NPR reports on a study giving volprioc acid to adults and training them on pitch (singing):
Brain plasticity is useful for a whole lot more than learning pitch. As the article notes it would be invaluable for training one's ear to pick up sounds of foreign languages, but also it seems reasonable to this commentator that high levels of plasticity during rationality training or other forms of self-development would result in more transformative results.