My strong suspicion is that children just have no responsibilities and are socially allowed to not be able to talk while everyone will speak at their level with a great deal of patience.
Perhaps coincidentally, he is now fluent in more languages than anyone else I personally know, and leveraged that into a consulting career.
We much overestimate how well kids learn, and how "easy" is for them. Many kids have language difficulties, and they usually know, and they don't feel too great about it.