people like to talk up how good babies are at language acquisition but I think they're mostly just stuck with thousands of hours of free time and literally nothing else to do. and even then, if you take me (a 30-something barely-bilingual adult) and stick me in a six-month immersion program for any language under the sun, I guarantee I'll come out able to hold a far more cogent conversation in that language than a six-month-old native speaker