In an effort to give kids a leg up in life, parents often enroll them in classes for learning a second language at an early age — and earlier and earlier age. It’s true that the younger children are, the better their language acquisition. But new research by Massachusetts Institute of Technology means we can all relax a bit: The study found children remain very skilled at learning the grammar of a new language for longer than expected – until around age 17 to 18.

However, the study also found that in order to gain native proficiency over a language, the ideal age to start learning it is younger.

“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,” says study author Joshua Hartshorne, an assistant psychology professor at Boston College who conducted the study as a researcher at MIT. “But we start seeing a decline after that.”

“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.”

Lab-based investigations of this subject have previously found that adults have an edge in language learning. But such studies likely do not accurately replicate the process of long-term learning, Hartshorne says.

To get a more accurate picture of the difference between language acquisition in kids and adults, researchers created a specialized quiz to test the grammatical proficiency of thousands of people who were all at different stages of learning a new language. The quiz was also designed to distinguish between English dialects, which have variance in acceptable grammar.

Within hours of being posted on Facebook, the quiz, “Which English?” had managed to go viral, reaching 670,000 participants and giving the team enough data to analyze and draw conclusions.

“We had to tease apart how many years has someone been studying this language, when they started speaking it, and what kind of exposure have they been getting: Were they learning in a class or were they immigrants to an English-speaking country?” Hartshorne explains.

They concluded that kids can swiftly learn a new language between ages 10 and 18, but because they have a smaller window of time to learn it before their learning ability declines than a younger child would, they won’t develop native-like proficiency.

The authors note that while adults are still great at learning foreign languages, they’re unlikely to reach the same proficiency as someone who has learned from childhood or adolescence. Researchers are still uncertain about the reason behind this difference.

“It’s possible that there’s a biological change. It’s also possible that it’s something social or cultural,” says Josh Tenenbaum, who is an MIT professor of brain and cognitive sciences and one of the paper’s co-authors. “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.”