Arturo E. Hernandez explores how age affects the ability to learn multiple languages:
Particularly sensitive to age is a person’s ability to speak without an accent and to detect speech sounds that are not present in their native language. For example, infants can detect sounds from a language not in their environment at six months of age. By 10 months of age they lose this ability. This suggests that the ability to detect speech sounds from around the globe is available to all infants but slowly fades away. Another arena where age plays a role is in the processing of grammar. Those who learn a second language later in life do not perform as well on tests of grammar as early learners. Hence, the ability to learn grammar and speech sounds appears to be very dependent on the age that one first learns a language.
Despite this general rule, there are some very interesting exceptions.
For example, Christophe Pallier and his colleagues tested a group of adults who had been born in Korea and adopted as children in France between the ages of 4-8. This group of adults were asked to listen to sentences in Korean, French, or an unknown language. The results revealed no difference in their brain activity when compared to native French speakers. That is, both groups showed similar activity for French, Korean, and a foreign language. Furthermore, the Korean adoptees had no discernible accent. They sounded French. The results are intriguing because they suggest that a language can be lost even relatively late in childhood. This suggests that the age at which a language is learned is not the only predictor of how well a language is spoken as an adult.
Previous Dish on the subject here.