A new study performed at MIT suggests that children remain very skilled at learning the grammar of a new language for much longer than expected—up to the age of 17 or 18. 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.

“If you want to have native-like knowledge of English grammar, you should start by about 10 years old. We don’t see very much difference between people who start at birth and people who start at 10, but we start seeing a decline after that,” says Joshua Hartshorne, an assistant professor of psychology at Boston College, who conducted this study as a postdoc at MIT.

People who start learning a language between 10 and 18 will still learn quickly, but since they have a shorter window before their learning ability declines, they do not achieve the proficiency of native speakers, the researchers found.

To attract enough participants who were in different stages of learning English—Hartshorne estimated they would need at least half a million—the researchers set out to create a grammar quiz that would be entertaining enough to go viral.

The test included questions that are likely to trip up non-native speakers, such as determining whether a sentence like “Yesterday John wanted to won the race” is grammatically correct. To entice more people to take the test, the researchers also included questions that were not necessary for measuring language learning but were designed to reveal which dialect of English the test-taker speaks. For example, an English speaker from Canada might find the sentence “I’m done dinner” correct, while most others would not. The 10-minute quiz “Which English?” attracted responses from more than 600,000 people.

Still unknown is why the critical period ends around age 18.

“It’s possible that there’s a biological change. It’s also possible that it’s something social or cultural,” says Josh Tenenbaum, PhD ’99, a coauthor of a paper on the study that appeared in Cognition, who is an MIT professor of brain and cognitive sciences (see “Boosting AI’s IQ,” Page 12). “There’s roughly a period of being a minor that goes up to about age 17 or 18 in many societies. After that, you leave your home, maybe you work full time, or you become a specialized university student. All of those might impact your learning rate for any language.”