回复：谢谢大家的回复，很有帮助

When a baby is born, s/he can distinguish lots more different sounds. While growing up and exposed to certain language environment, s/he will lose the ability to distinguish certain sounds that s/he does not hear, e.g., some southerners cannot distinguish between Nan and Lan. The younger a child starts speech therapy, the more likely s/he can re-acquire the ability to distinguish the specific sounds.