Scientists Pinpoint Best Age to Learn a Second Language

Updated | Children must start to learn a new language by the age of 10 to achieve the fluency level of a native speaker, a new study has suggested.

Evidence indicates it becomes harder to learn a language other than our mother tongue as we progress through adulthood. Now, a new study suggests that children are highly skilled at learning the grammar of a new language up until the age of 17 or 18, much longer than previously thought.

But while children will continue to learn quickly past the age of 10, it is unlikely they will become fluent in the new language. Scientists believe this is because they have a smaller time frame before their learning abilities begin to weaken around 17, compared with those trying to pick up the same skills before 10.

Joshua Hartshorne, an assistant professor of psychology at Boston College who authored the study as part of his postdoctoral qualification at MIT, said the team was surprised that a language could largely be mastered until the teen years. He told Newsweek that the findings have wider implications for understanding the mind and the brain.

"[In the past] researchers suggested that changes that happen in the brain at around four to five-years-old might be the culprit in our declining ability to learn language. Others focused on changes that happen at puberty. Our finding that the ability to learn language is actually preserved up until early adulthood throws a wrench in that whole discussion," he said. "It means we may need to go back to the drawing board in trying to explain why adults have trouble learning language."

To obtain their findings, which were published in the journal Cognition, researchers at MIT and Harvard studied the results of a grammar quiz completed by almost 670,000 participants, the biggest ever data set on language and learning.

Past studies have struggled to shed light on the topic as adults and children were tested in lab settings where the former were more likely to be comfortable, thus skewing the results. Instead, researchers documented the grammatical proficiency of people of different ages who started learning English at different points in their lives.

The resulting quiz—called Which English?—was designed to challenge non-native speakers. Participants were asked to state whether sentences such as "Yesterday John wanted to won the race" were incorrect. In an attempt to make the test go viral and collect more respondents, the researchers also added questions relating to the grammatical quirks of different English dialects.

Participants were asked to include information such as their age when taking the quiz and the age at which they started to learn English.

"It's been very difficult until now to get all the data you would need to answer this question of how long the critical period lasts," said Josh Tenenbaum, an MIT professor of brain and cognitive sciences and an author of the paper. "This is one of those rare opportunities in science where we could work on a question that is very old, that many smart people have thought about and written about, and take a new perspective and see something that maybe other people haven't."

Further research is now needed to understand why our ability to learn a new language appears to drop past a certain age.

"It's possible that there's a biological change. It's also possible that it's something social or cultural," Professor Tenenbaum said.

"There's roughly a period of being a minor that goes up to about age 17 or 18 in many societies," he continued. "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."

Professor Hartshorne also emphasized that the study is unique because of its scope.

"We tested English-speakers in nearly every country on Earth," he told Newsweek. "And although we didn't focus on it in this paper, we learned a lot about how English varies from place to place and dialect to dialect. It turns out that the grammars of the world Englishes vary in complex and interesting ways."

This piece has been updated to include comment from Professor Joshua Hartshorne.