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.
Chemistry aside, my experience is that a family must have a HIGH culture before the child can be truly bilingual.
Maybe so. But it is always possible to learn a new language. I am STUMBLING my way through learning Italian, and I had Latin and have a degree in Spanish. It stimulates the brain when you bother to learn a new language. I am working hard on pronunciation as I learn the new words. The woman sitting next to me in Italian class yesterday is pronouncing the words like English. She isn't even TRYING. I won't sit by her next time. LOL She kept throwing me off.