This lab experiment at the University of British Colombia’s baby speech lab in Vancouver shows infants under 10 months old differentiating between phonemes that sound identical to any adult who doesn’t speak Hindi.
So what about the 12-month-old? Has she already lost her chance to be fluent in a second language? A child in grade school who wants to learn Spanish for the first time will have to relearn how to roll her Rs and stop using her teeth when she says Vs, but she will still have the potential to re-learn them well enough to sound native. The double-edged sword here is that a child who was fluently bilingual as a baby can completely forget one of the languages by adolescence, phonemes and all, if it’s not reinforced along with the language spoken by her peers, which will automatically become her native language. And no pressure, but this is her last chance to ever sound native at a second language. The more languages she speaks before she hits adolescence, the better she will be at learning new languages even after passing the final critical period of language development at puberty.
How do babies acquire such language-specific sounds? The TED Talk describes lab experiments like the one above as well as brain scans that demonstrate how babies observe the world around them to determine their cognitive and linguistic needs.