The concept of a critical period is well in nature. In human beings there seems to be a critical for the first language acquisition. Research shows that any human who is not exposed to any language before puberty, becomes completely unable to the syntax of their first language later in life. From my personal experience I have learnt that any individual who learns their first language at infancy and later (at puberty) gets to move from their motherland to foreign land where no one speaks their language, no matter how long these individuals stay from home they can never forget their mother tongue.
Accent may change but they will always remember their first language Here is another practical example to prove that there is a critical period in language development. I happen to be living in Africa; my bosses are Italians (man and wife) they have had to learn English and Kiswahili for easy communication with the locals. They have a five year old baby. When the baby is with her parents they always speak in their first language; when she is left behind with the nanny she is spoken to in Kiswahili all the time.
The nanny knows no other language apart from her mother tongue and Kiswahili. This baby has been looked after by this same nanny since she was born. At the age of three a teacher from America was employed to teach this girl. She knew no other language but English. Now look at this closely; the baby is now very fluent in two languages; her first one Italian and her second one; Kiswahili. She is so fluent that she corrects her parents on it!
She seems to be doing just fine in English for the last two years she has been taught. Interestingly her teach who is very interested in learning Kiswahili asks her enough times to translate to her in English what has been said in Kiswahili by colleagues. I would say that all depends with the stage at which the language was introduced to an individual. The stage in life. • Kiswahili is a language for the East African state and some of the West African.