They say you can't teach an old dog new tricks, and apparently that's definitely true when it comes to humans and language.
A new study finds that the ability for people to learn new languages begins declining at the age of 18. The study was based off a Facebook language test taken by nearly 670,000 people, which is considered one of the largest linguistic tests in history. In addition to the language learning ability declining after the age of 18, they also found that a person's chances of becoming fluent increase if they begin studying before the age of 10.
The researchers didn't determine why a person's language learning ability declines after 18, although they hypothesized that it's because the brain becomes less adaptable after that age. Therefore, adults are more likely to be "saddled with an accent and conspicuous grammatical errors."
Basically, we've been using one language for so long our brains can't really switch our accent and grammar rules off to learn a new one.
That's not to say it's impossible for adults to learn a new language. Other studies show dedicated language learners can pick up a new one late into adulthood, and even this new study shows people who are immersed in an area where they speak a new language are more likely to pick it up. There's also the benefit that learning a new language can help stave off a decline in brain function later in life.
(h/t Mental Floss)