Why is it easier for children to learn an additional language at an earlier age?

Answer 1

There is something biological going on that we still don't completely understand. About puberty, the easy language learning part of the brain switches to other activities--socialization and "breeding" behavior. If that language-learning part of the brain has been regularly stimulated throughout childhood with multiple languages, there is evidence that it does not switch over to other behaviors at puberty, but continues to allow easy language learning through life. However, for most people, only one language has been learned (the native one) with less and less use being made of the language learning section as the child ages. Therefore, at puberty, that seldom used part of the brain is co-opted for the "more important" activity of copying one's genes into the next generation.