Dana G. Smith in Scientific American:
The older you get the more difficult it is to learn to speak French like a Parisian. But no one knows exactly what the cutoff point is—at what age it becomes harder, for instance, to pick up noun-verb agreements in a new language. In one of the largest linguistics studies ever conducted—a viral internet survey that drew two thirds of a million respondents—researchers from three Boston-based universities showed children are proficient at learning a second language up until the age of 18, roughly 10 years later than earlier estimates. But the study also showed that it is best to start by age 10 if you want to achieve the grammatical fluency of a native speaker.
To parse this problem, the research team, which included psychologist Steven Pinker, collected data on a person’s current age, language proficiency and time studying English. The investigators calculated they needed more than half a million people to make a fair estimate of when the “critical period” for achieving the highest levels of grammatical fluency ends. So they turned to the world’s greatest experimental subject pool: the internet.