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