Variance in language-learning ability after the so-called “critical period” can be to large extent explained by increase in its optionality. As a child you rea

Variance in language-learning ability after the so-called “critical period” can be to large extent explained by increase in its optionality. As a child you really have no option but to learn your first language. The entire environment around you screams “learn the language” every hour of every day. People around you are incessantly encouraging you to learn it. When you’re an adult, it’s very optional for obvious reasons. In fact, certain social dynamics might actively discourage one from learning a new language. (Bear in mind the difference between first- and higher-order volition. The former does not readily lend itself to conscious control and is probably impossible to measure—merely spending large amounts of time on learning a language is not a reliable indicator.)