When is the best time for a child to start learning a foreign language?

Most people believe that learning a language - mother tongue or foreign - should start as early as possible. Little children pick up whatever language they hear, and speak it naturally without knowing whether it is a native or foreign language. They don't have to distinguish the difference or analyze sentence patterns. They just speak it for the sole purpose of communication. Therefore, the best time for a child to learn a language is when he/she starts to talk. However, for non-native English speaking children, it is hard for them to get that kind of English speaking environment. Hence, they have to go to school to receive formal, systematic education so that they can learn efficiently in a short time.