But the cut-off age appears to be 17 or 18, much later than linguists previously thought.
According to the team at MIT which ran the study, however, native speaker-like grammar competence is rare among learners who started learning after the age of ten.
The study is based on the results of a specially designed internet grammar test.
The MIT study used a grammar quiz to create a snapshot of grammatical proficiency among learners of all ages and all language levels. The quiz, which was designed to go viral, was taken by 669,498 people after being posted on Facebook.
After taking the quiz, respondents were asked to give their age, the age at which they started learning English and other information about their language learning background.
'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?' team leader Joshua Hartshorne told the Medical Xpress.
Having developed and tested a variety of computational models to see which was most consistent with their results, MIT found that the best explanation for the data was that grammar-learning ability remains strong until the late teens.
Second language acquistion theorists have long down played the existence of a critical age theory, particularly around grammatical acquisition.
It is however generally accepted that the ability to recognise and reproduce L2 phonemes is extremely strong in small infants but generally declines as children get older.
The MIT paper, which is co-authored by celebrity psycholinguist Professor Steven Pinker, comes out today (May 1) in the latest edition of Cognition.
Pic: Rebecca Goldstein