The window to learn a language's grammar stays open until about 17

There really does seem to be a critical period for picking up a new language — but it closes later than the old textbooks claimed. In 2018, researchers from MIT, Boston College and Harvard, led by Joshua Hartshorne, ran a viral online grammar quiz and analysed responses from nearly 670,000 English speakers, the largest sample ever used for the question.

By disentangling a person’s current age, the age they first learned English, and their years of practice, the model showed that the ability to absorb a new language’s grammar stays remarkably strong through the teens, only beginning a steady decline around age 17-18.

The window stays open longer than thought — but reaching truly native-like fluency still favours an early start.

The finding reframes a long debate. Adults are not doomed to fail at languages; the steep drop-off comes later than feared. But to end up indistinguishable from a native speaker, the data suggest beginning in childhood remains the surest route.