The human brain forms a ‘language area’ when the brain is young and malleable. Which also means that if we do not learn speech when we are young we will never learn to talk. (Children kept in isolation without any contact was proof to this). And if you don’t use it you lose it.
Some neurologists and other scientist also have a theory that if you learn to speak a second language then learning a third, a fourth and so on is easier accomplished later on in life.
Now this fairly interesting article here http://www.nytimes.com/2012/03/18/opinion/sunday/the-benefits-of-bilingualism.html?_r=1&ref=global-home
Which states ‘Bilingualism’s effects also extend into the twilight years. In a recent study of 44 elderly Spanish-English bilinguals, scientists led by the neuropsychologist Tamar Gollan of the University of California, San Diego, found that individuals with a higher degree of bilingualism — measured through a comparative evaluation of proficiency in each language — were more resistant than others to the onset of dementia and other symptoms of Alzheimer’s disease: the higher the degree of bilingualism, the later the age of onset.’