Yesterday, I got into a little discussion about how fast children learn languages. Lost of people seem to be amazed by the fact that a four year old speaks Chinese. But if you think about it, this isn't very spectacular. I can learn a language pretty well in four years, especially if I don't have to do anything else at the same time. And after four years, children still don't speak very well.
Of course it is amazing that a baby can learn a language, small and helpless as it is and children do in the end learn the language better than I will ever will, but they don't learn very fast.