It’s long been speculated that there’s a critical period for learning a new language, but the specifics are a matter of debate. It is difficult to follow a sufficient number of language learners through their years of learning. But a new study has got over that difficulty by using the ability of Facebook to get vast numbers of people, who represent many stages of learning.
The study involved a 10-minute quiz on English grammar, after which users were asked to reveal their current age and the age at which they began learning English, as well as other information about their language background. Complete data was received from 669,498 people (both native and non-native English speakers).
Testing a number of computational models to see which was most consistent with the results, the researchers concluded that grammar-learning ability remains strong until age 17 or 18, at which point it drops. This is a much longer period than previously thought.
However, the study also found that it is nearly impossible for people to achieve proficiency similar to that of a native speaker unless they start learning a language by the age of 10. There wasn’t much difference between those who started learning the language at birth and those who began at 10, but if you start learning after 10, you don’t have a long enough time before reaching 18, to achieve the proficiency of native speakers.
It’s not clear, however, that these differences necessarily have to do with physiology. It still may be social or cultural, given that people’s lives customarily change at that age. Perhaps it’s simply a matter of how much time and effort you are able to devote.
The quiz was designed to be entertaining enough to go viral (in which it succeeded!), and included questions that used grammatical rules most likely to trip up a non-native speaker, as well as questions designed to reveal which dialect of English the test-taker speaks (acceptable grammar can vary by dialect).
Do note that this is only about learning to the standard of a native speaker! It emphatically does not mean that you can’t learn a language as an adult!
 . A critical period for second language acquisition: Evidence from 2/3 million English speakers. Cognition [Internet]. 2018 ;177:263 - 277. Available from: http://www.sciencedirect.com/science/article/pii/S0010027718300994