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Languages survive through babies, not adults
The age curve
Skill at picking up a second language stays at its peak for babies and children until they turn seven. After that it declines steadily, and after puberty it falls off the map. No scientists dispute this curve, though laboratories around the world are working out why it looks the way it does.
Koro is a newly discovered language spoken by about 800 people in India. The mothers who speak it talk to their babies in it, because that is how the language will be preserved. Speaking Koro to adults would not do the job, since adult brains no longer absorb a language the way infant brains do.
Why learning slows
One explanation comes from the same sound statistics that babies collect. Adults are governed by representations in memory formed early in development. A mathematical argument holds that language learning may slow down once those distributions stabilize.
That view also raises questions about bilingual people, who must keep two sets of statistics in mind at once and flip between them depending on who they are speaking to.
How do researchers know learning declines with age?
A well-known 1989 study by Jacqueline Johnson and Elissa Newport tested Chinese and Korean immigrants to the United States on English grammar. Those who arrived before about age seven scored like native speakers. Scores fell steadily for those who arrived later, and people who arrived as adults showed the widest spread. All of the participants had lived in the country for at least five years.
Why do endangered languages depend on children?
Linguists treat the handing down of a language from parents to children as the clearest sign of its health. UNESCO's framework for language vitality lists intergenerational transmission as its first factor. A language spoken only by adults will shrink as those adults die. A language that children learn at home can continue even with a small number of speakers.
How does the bilingual brain switch between languages?
Experiments on word recognition show that both languages of a bilingual person are active at the same time, even when only one is being used. Brain imaging studies point to frontal regions, including the prefrontal cortex and the anterior cingulate, as helping to select the right language and hold back the other. This constant selection is one reason researchers have studied bilingualism as a form of mental exercise. The details of how much it matters remain under active debate.