The child's brain is different from the adult brain in that it is a very dynamic structure that is evolving. A two-year-old child has twice as many synapses (connections) in the brain as an adult. The young brain must use these connections or lose them. Babies are born "citizens of the world" in that they can distinguish differences among sounds borrowed from all languages. They are ready to learn any language they hear. Studies show that in language development there is a window of opportunity in which the child learns languages normally. After this period, the brain becomes slowly less plastic and by the time the child reaches adolescence, the brain cannot develop "richly and normally any real cognitive system, including language." ...The power to learn language is so great in the young child that it doesn't seem to matter how many languages you seem to throw their way...They can learn as many spoken languages as you can allow them to hear systematically and regularly at the same time. Children just have this capacity. Their brain is just ripe to do this...there doesn't seem to be any detriment to...develop several languages at the same time.
- National Network for Early Language Learning