I tend to agree with Caesar. While children do often pick up language fairly well, they're under very different pressures than adults to do so. A child between 0-5 is pretty much concentrating on language learning. A 30-year old adult probably has a job. Also, we were spending a lot of time in the car a few months ago, so we bought a CD on learning Chinese. Spoken Chinese is not very similar to spoken Japanese, so my kids and I were on a pretty even starting level (my wife was not -- since Japanese has fewer actual sounds in it, distinguishing some different sounds is still difficult for her). Now it's several months later, and I'm certainly as competent as my kids in Chinese (which isn't to say I know much -- basic introductions and such -- but I've retained more than all of them despite having less listening time. I'm pretty good at languages, but my oldest is probably pretty close and has a steeltrap memory*). There's a lot that's not known about the brain, though, so I'd say the jury's still out. I'd also add that a lot of kids who grow up multilingual, even while completely fluent, are sometimes behind in areas where only people who know them intimately over the long term (i.e., parents) can perceive. My kids all went to Japanese preschool, and live here in the US. My oldest did not receive any ELL support when he went to public Kinder; they viewed him as fluent. Nevertheless, I knew he wasn't quite where he "should" be given his native ability, and knew that if he were monolingual he would be more advanced. Even now he's better than average, but I know the lag had an effect (it doesn't bother me because I have no reason to believe it's a permanent effect, and even if it were the benefits of being multilingual outweigh any negatives). * my advantages over the oldest are better focus and the use of downtime practice.