I presume you mean they spoke no English before they moved to the USA. I recently read Steven Pinker's book The Language Instinct, which left me with the impression that what you describe is impossible, and that adults are never able to learn a language to 'native' proficiency. This article seems to refute that.
^ Basically the above study, sponsored by the NIH, found that when you correct for all relevant factors the critical period signal not only disappears but becomes anti-correlated: kids have a HARDER time in all aspects of language learning than adults, when the same level of interest and effort is applied.
That's, IMO, an unwarranted (and, taken strictly, almost implausible) assumption. (Especially on the interest side.)