So, what is the special secret in language processing during the first years of life? Or conversely, why do adults have such a hard time learning languages? The short answer is brain development -- and the unfortunate lack thereof in adults.
Baby's special linguistic brain advantages come in three varieties: pure mental computation (number of synapses), brain plasticity, and sound perception. All of these are overlaid with the inherent human trait of acquiring language as soon as possible.
The child's synapses peak before the age of two. Almost all neurons are formed in utero and then move to their respective destinations. Throughout the first year the neural junctions (synapses) continue to increase rapidly establishing long distance connections by around 9 months. Brain metabolism reaches adult levels at around ten months and then rapidly exceeds it. A two-year old child actually has fifty percent more synapses than an adult. However, this growth spurt in the brain clearly can't go on forever, and when it levels off, the brain functionality has been shaped by the early experiences.
Secondly, aside from being proverbial sponges, infants are more perceptive than adults, especially in the realm of sound perception. Infants are infinitely better at detecting different sounds and hearing the nuances in foreign languages with perfect clarity and precision. These individual sound elements are called 'phonemes,' and decades of scientific studies have provided overwhelming evidence that adults perceive the same sound differently than infants. Once sounds have proven useless in our native language(s), our brains start to prune them out, leaving behind only the top 40 or so. When we're introduced to new foreign sounds later in life, we simply cannot hear the difference. And, what we can't hear, we can't imitate.
In an experiment using a speech synthesizer, the sound /b/ was gradually changed into a /p/. Speakers of many languages were tested. English, French and Spanish speakers all heard two categories, but they delineated the shift at a different place along the continuum. Thai speakers heard three categories! The various listeners also could not hear the gradual changes, but only a clear and sudden leap from a /b/ to a /p/. Their brains had created a finite number of 'sound buckets' or filters at an early age. That limited sound inventory helps us to either ignore sounds that don't make sense (the ones we don't use in our immediate surroundings) or to approximate them to the closest match we can dig out of our brain's sound assortment.
Thirdly, brain plasticity refers to the brain's ability to rewire itself to adapt to new experiences -- in other words the flexibility of the brain. Plasticity in regards to language has been most dramatically demonstrated in children who have had a portion of the brain or the entire left brain removed (the left brain is the normal position for the language center.) These children almost completely recouped their language abilities using the right hemispheres of their brains -- provided the procedure took place before the age of four or five years. Other neurons were able to come to the rescue, taking over the duties of the ones removed. Patients undergoing these operations during puberty, however, were destined to a life with either language impairment or a complete lack of language.
Plasticity also effects grammar and sound perception far more than the ability to learn vocabulary. Any adult who has tried to learn a new language knows it's easier to pick up words than it is to learn grammatical rules or to pronounce the words flawlessly.