Language learning and youth study

Scientists have long posited that there is a "critical period" for language learning, but new research suggests that the time frame stretches on much longer than previously thought. The study suggests that children remain skilled at learning the grammar of English up to the age of 17 or 18. The study also found that it is difficult for people to achieve proficiency in English similar to that of a native speaker unless they start learning a language much earlier, by the age of 10. People who start learning a language between the ages of 10 and 18 will still learn quickly, but since they have a shorter window before their learning ability declines, they're less likely to reach the proficiency of native speakers, the researchers found.
Hartshorne, J. K., Tenenbaum, J. B., & Pinker, S. (2018). A critical period for second language acquisition: evidence from 2/3 million English speakers. Cognition, 177, 263-277. http://l3atbc-public.s3.amazonaws.com/pub_pdfs/JK_Hartshorne_JB_Tenenbaum_S_Pinker_2018.pdf