It turns out that babies have only their first year to be native speakers of any languages. Once they pass that period, their language hearing abilities will be set or culture-bound. From then on, it will only get harder and harder to learn new languages.
Also, having real people talk in front of the babies yield much better learning results compared to using audio and video devices. This finding suggests that we should spend more time with our babies so they can pick up the language early on.
Patricia Kuhl: The linguistic genius of babies