If you bring adults and children into the lab and try teaching them a new language, adults will learn much more of the language much more rapidly than the children. This is odd, because probably one of the most famous facts about learning languages -- something known by just about everyone whether you are a scientist who studies language or not -- is that adults have a lot less success at learning language than children. So whatever it is that children do better, it's something that operates on a timescale too slow to see in the lab.
This makes studying the differences between adult and child language learners tricky, and a lot less is known that we'd like. Even the shape of the change in language learning ability is not well-known: is the drop-off in language learning ability gradual, or is there a sudden plummet at a particular age? Many researchers favor the latter possibility, but it has been hard to demonstrate simply because of the problem of collecting data. The perhaps most comprehensive study comes from Kenji Hakuta, Ellen Bialystok and Edward Wiley, who used U.S.A. Census data from 2,016,317 Spanish-speaking immigrants and 324,444 Chinese-speaking* immigrants, to study English proficiency as a function of when the person began learning the language.
Their graph shows a very gradual decline in English proficiency as a function of when the person moved to the U.S.
Unfortunately, the measure of English proficiency wasn't very sophisticated. The Census simply asks people to say how well they speak English: "not at all", "not well", "well", "very well", and "speak only English". This is better than nothing, and the authors show that it correlates with a more sophisticated test of English proficiency, but it's possible that the reason the lines in the graphs look so smooth is that this five-point scale is simply too coarse to show anything more. The measure also collapses over vocabulary, grammar, accent, etc., and we know that these behave differently (your ability to learn a native-like accent goes first).
A New Test
This was something we had in mind when devising The Vocab Quiz. If we get enough non-native Speakers of English, we could track English proficiency as a function of age ... at least as measured by vocabulary (we also have a grammar test in the works, but that's more difficult to put together and so may take us a while yet). I don't think we'll get two million participants, but even just a few thousand would be enough. If English is your second (or third or fourth, etc.) language, please participate. In addition to helping us with our research and helping advance the science of language in general, you will also be able to see how your vocabulary compares with the typical native English speaker who participates in the experiment.
Hakuta, K., Bialystok, E., & Wiley, E. (2003). Critical Evidence: A Test of the Critical-Period Hypothesis for Second-Language Acquisition Psychological Science, 14 (1), 31-38 DOI: 10.1111/1467-9280.01415
*Yes, I know: Chinese is a family of languages, not a single language. But the paper does not report a by-language breakdown for this group.