All babies are born with the innate ability to acquire language and because of this ability do so at a rapid rate. Children are able to hear and understand reasonably complex structure or patterns without ever having a direct lesson in grammar or speech.
However, it’s commonly believed that there’s a certain period of “linguistic plasticity” that extends only to a certain age (before the age of 8 years at the outside) after which language learning becomes much more difficult and less successful. Studies of so-called “wolf children” who, for various reasons, were not exposed to language before the age of 8, have shown that these children have very limited success in acquiring language thereafter – especially grammar.
One famous example of this is the case of 13 year old Genie, who was discovered isolated in a room by her father. Although, after her discovery she was subsequently able to learn the words for identifying objects, all attempts to teach her grammatical English – how to use the words in comprehensible and grammatically correct sentences – met with failure.