A child's brain differs to an adults because it is developing so rapidly. The young brain must use these connections or lose them. It makes sense to take advantage of this time to give the child as much knowledge as possible - a second language is the ideal medium. In addition, when considering the best age at which to start this extra language training it's important to remember one very important factor. Learning to talk does not begin with the child's first words, but long before that.
Understanding is always one step ahead of talking.
If we are learning a foreign language then we find that we can understand what is being said way before we are able to formulate phrases and actually talk in that language ourselves. With babies, it is generally accepted that their understanding is about six months ahead of their speech. Therefore, the younger the better - in theory. Ideally, a good age to start formally teaching a young child is probably around 6 months or more. At this age they are taking some notice of their surroundings and can usually sit up.