A lot of people feel that it’s much harder to learn a foreign language as you grow older — and they’re right. A new study found that if you want to achieve the proficiency of a native speaker, you’d best start when you’re 10 years old.

Image credits: US Embassy in Canada.

There is a lot of evidence that shows children find it much easier than adults to learn a foreign language, which has prompted researchers to suggest that there is a “critical period” for language learning. But until now, no one was really sure what that age period is, and what is its underlying mechanism. Now, MIT researchers believe they have solved one of those things.

They found that the window starts to close when you’re about 10 years old. Although children up to 17 or 18 learn a new language faster than adults, and their ability declines slower than in adults, it becomes very difficult to reach the proficiency of a native speaker if you start later than age 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.

The findings are based on an analysis of a grammar quiz taken by nearly 670,000 people — the largest dataset of its kind. Following people as they learn a new language (or forget what they once learned) is a tedious task, so instead, researchers tried something else: they took snapshots of hundreds of thousands of people who were in different stages of learning English. They also used a game called “Which English?” to assess 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. Within hours after being posted on Facebook, the 10-minute quiz “Which English?” had gone viral.

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They knew they needed at least half a million participants, so they found a way to make it work. This is how they gathered this unprecedentedly large dataset.

“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,” says 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.”

While it was common knowledge that children are much faster at learning foreign languages, narrowing down the specific interval is very important, for instance, for deciding when kids should start learning a new language. Researchers found that the grammar-learning ability remains strong until age 17 or 18, after which it starts to decline — which was rather unexpected.

“It was surprising to us,” Hartshorne says. “The debate had been over whether it declines from birth, starts declining at 5 years old, or starts declining starting at puberty.”

However, it’s not clear why the decline starts to happen. It may either be a cultural factor (as they become adults, children might tone down their learning rate) or a biological one, as we already know that the brain starts to lose some of its plasticity around that age.

Still, you shouldn’t let this discourage you. Even if it’s a bit harder (or even because it’s a bit harder), learning a foreign language can be very rewarding — and also useful. It’s not just the ability to talk to someone from another country, studies have shown that learning a second or third language does wonders for your brain and mental health. Furthermore, some research suggests that even when you’ve forgotten a language, you haven’t really forgotten it.

The study has been published in the journal Cognition.