Note also that while kids will pick up languages faster, more spontaneously, and with better results (the ability to pick up a flawless accent and perfect command of finer points of grammar usually disappears in late childhood), they will also forget them unbelievably quickly and thoroughly without active use. As an adult, your command of a language may get rusty, but it will never fall to zero as long as your brain is functioning decently. On the other hand, kids who change environments may forget even their first native language so thoroughly that they'll be barely able to recall a single word.