|Learning a Foreign Language
A child growing up in a neighborhood filled with many languages will learn those languages fluently and effortlessly. The brain has no nationality. The neurons don't know that they are learning English or French or Japanese, they simply learn what they hear. Immigrant parents and grandparents are excellent sources of foreign languages; my wife calls this approach the "grandmother tongue."
After age 10, because the grammar area is mostly finished, it is very difficult to learn other languages fluently. Fluency can be more or less defined as the ability to think in a language without translating.
From the brain's point of view, learning foreign languages in high school is fighting the brain's natural order of learning. Not only is the brain grammar area mostly finished, but the natural dialogue needed to make a language fluent is prevented because the students are not supposed to talk with their friends in class. Like pouring water onto a duck's back -- it is mostly wasted effort. Both teachers and students struggle and get frustrated. To survive, the brain uses bandaid solutions such as translating vocabulary, and memorizing rules of grammar.
In contrast, putting foreign language into kindergarten and primary school is fun for everyone, because the brain is doing what it loves to do -- learning the right material, at the right time, in the right way. To encourage fluency, students in immersion are encouraged to talk a lot with each other.