There is a critical cut-off age for learning a language fluently, according to research.
If you want to have native-like knowledge of English grammar, for example, you should ideally start before age 10, say the researchers.
People remain highly skilled learners until 17 or 18, when ability tails off.
The findings, in the journal Cognition, come from an online grammar test taken by nearly 670,000 people of different ages and nationalities.
The grammar quiz was posted on Facebook to get enough people to take part.
Questions tested if participants could determine whether a sentence written in English, such as: "Yesterday John wanted to won the race," was grammatically correct.
Users were asked their age and how long they had been learning English, and in what setting - had they moved to an English-speaking country, for example?
About 246,000 of the people who took the test had grown up speaking only English, while the rest were bi- or multilingual.
The most common native languages (excluding English) were Finnish, Turkish, German, Russian and Hungarian.
Most of the people who completed the quiz were in their 20s and 30s. The youngest age was about 10 and the oldest late 70s.
When the researchers analysed the data using a computer model, the best explanation for the findings was that grammar-learning was strongest in childhood, persists into teenage years and then drops at adulthood.
Learning a language is often said to be easy for children and to get more difficult as we age.
But late learners can still become proficient, if not seamlessly fluent, say the researchers.
It is unclear what causes the drop in the optimal learning rate seen at about age 18. The researchers suggest it could be because the brain becomes less changeable or adaptable in adulthood.
Study co-author Josh Tenenbaum, a professor of brain and cognitive sciences at the Massachusetts Institute of Technology in the US, said: "It's possible that there's a biological change. It's also possible that it's something social or cultural.
"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 specialised university student. All of those might impact your learning rate for any language."
That doesn't mean learning another language in adulthood is futile.
Learning another tongue is said to be good for your brain and might even delay the onset of dementia, according to some studies.
Prof Marilyn Vihman, from the University of York's department of language and linguistic science, said: "The suggestion that you can't reach native-like ability if you don't start early is questionable.
"Such cases are rare, but they do occur and are documented.
"There are cases of people in their 20s who learn a new language and can pass as spies.
"There are two, three or four documented cases like that.
"I don't think there is a critical age as such, just a plateau that sets in after the teen years for most but not all speakers."
Dr Danijela Trenkic, also from the University of York, pointed out that the study dealt with only one aspect of language - grammar.
"You can be an excellent communicator, even if you don't sound like a native speaker or don't get all your sentences grammatically correct."