A suggests that it is to start learning a language before the of ten. Researchers said the to learn grammar well is up until the age of 18. This is the of the "critical " when language-learning ability starts to decline. A researcher said children easily become because, "that's when you're best at learning languages". He said it's to become truly bilingual later in .
The research was based on 670,000 grammar quizzes. The had grammar designed to confuse non-native . These helped to measure the language of people who started learning a language at ages. One researcher suggested older people might be too to learn a language because they leave home, go to university or start . All of affect language-learning ability.