Well, the aptitude children have for obtaining their native language dries up before puberty, normally. Some people are lucky enough to retain it, you send them into a foreign country for six weeks and they return talking like a native. At university, I had a professor with that talent. He spoke the usual languages plus Arabic, Gaelic... But that didn't make him any less boring as a teacher.

I recently read that kreol languages, which develop from pidgin languages after one generation*, tend to develop a system of perfective vs. imperfective verbs just like Russian has, even if the languages which were parents to the pidgin don't have that feature. That means such a feature seems to be part of the innate grammar we are all born with. Well, the way I am having a hard time with Russian perfective verbs shows me I have not retained that talent from childhood.

* Does that need some explaining? Don't want to wast more off-topic space if everyone knows what that means.