I saw it on Dateline or 60 minutes awhile back. But from a quick google search::
https://www.scientificamerican.com/a...ker-disappear/
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The older you get the more difficult it is to learn to speak French like a Parisian. But no one knows exactly what the cutoff point is—at what age it becomes harder, for instance, to pick up noun-verb agreements in a new language. In one of the largest linguistics studies ever conducted—a viral internet survey that drew two thirds of a million respondents—researchers from three Boston-based universities showed children are proficient at learning a second language up until the age of 18, roughly 10 years later than earlier estimates. But the study also showed that it is best to start by age 10 if you want to achieve the grammatical fluency of a native speaker.
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so to basically achieve FrostKing's level of goals, you pretty much need to start by age 10, and it's very difficult to be proficient if you start a brand new language after age 18
It's one thing to learn and memorize a few words, like " o" or "I'm hungry" or "I love you" in any X amount of languages. But to be able to listen and understand on the fly like a native speaker is very hard for adults who no longer have the brain malleability of a youth.