The critical period
There’s a hypothesis that we have a critical period for acquiring languages during our childhood, and that learning a language in later life, roughly after the age of 12 or 13, is difficult because of this. As a result of this theory, it’s widely believed that the earlier you start learning a foreign language, the more successful you’ll be.
According to an article I came across today, the different aspects of language acquisition take place at different times and rates. If there is a critical period, there probably isn’t one single one but many. We continue to improve our knowledge of our language(s) throughout our lives.
The article suggests that one reason why most of us find it difficult to learn new languages is because our brains have are set up to handle the language(s) we already know, and find other languages challenging, especially ones that differ significantly from our native ones.
The conclusion is that our language learning abilities decline with age, so the earlier you start learning languages the better, but “there is no particular age beyond which the effort is hopeless”.