In my (admittedly) very little experience, it seems kids learn faster, but only because they have to... Most of them have to go to school still, and are subjected to it on a daily basis for many hours. I mean, say a family has two parents and two kids (that all speak no or little of the new language). The parents will most likely look for jobs they can use their current language, for instance, a Japanese person getting a job in a sushi bar where 9 out of 10 of the employees speak Japanese. But the kids have no choice but to learn the new language, as it is all that is spoke in school.
I am seeing a Bulgarian woman now that moved here with her parents and brother 5 years ago. Her mom is retired, and her dad is a truck driver, and they both speak only basic English, but her and her brother speak very fluent English, I can speak as fast as I want, or tell the lamest jokes, and she understands everything, which I wasn't expecting when I first talked to her.
Now, if you'll excuse me, I have to write back to her!
-Fantom