A great deal of evidence suggests that it is more difficult to learn a new language as an adult than as a child, which has led scientists to propose that there is a “critical period” for language learning. However, the length of this period and its underlying causes remain unknown.

A new study performed at MIT suggests that children remain very skilled at learning the grammar of a new language much longer than expected — up to the age of 17 or 18. However, the study also found that it is almost 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