There is nothing in the brain making it impossible for an adult to learn to speak a second language well, as has been implied.

Just to clarify my position.

1. Before some magic number age (the one I remembered was age 7) humans learn language natively and without study.

2. After that age, some may acquire a language, by hard study and over a period of years much greater than was needed before the magic number age. Their fluency is not native, but may be quite good. These people are few and far between.

3. It is not impossible to learn a language after one's youth. It's simply tougher and the process of acquisition is different.

Ceci n'est pas un seing.