I used to teach school 1973-1975 in Texas. I remember, for example, a student named Gilbert. CUTE little kindergarten boy, who had just moved from Mexico. Didn't speak a word of English other than CAR! He loved American cars. I saw him a year or so later and he was 100% totally fluent in English. It works when in total immersion. I am moving to Italy soon and I am taking Italian NOW. I don't even visit another country without knowing basic conversation!
I read a piece in the Scientific American some time ago which addressed the "neurobiology" of language acquisition. The piece essentially said that the brain of a prepubescent child is somehow chemically attuned to learning languages but that something happens to the brain during/after puberty which changes that substantially.