Use it, or lose it. In Detoxing Childhood, Sue Palmer says " If neurons in the brain don't fire and wire, they'll eventually disappear. A baby is born with all the neurons it needs to speak any language on earth, but the sounds that the baby will learn are the ones that they hear regularly. The neurons for picking up other language sounds die away, so that after the age of six the brain can no longer discriminate them. This is why when Chinese people learn English later in life they find it so difficult to pronounce the "r"."