Most people believe that an adult learner can't hope to replicate the fluency of someone who learned another language in childhood. And certainly there is research to support this. However, people tend to confuse these findings - that the age of acquisition affects your representation of grammar - with the idea that children can learn words vastly quicker than adults. This is not true. Adults have a number of advantages over children:
- they usually have more and practiced strategies available to them,
- they have a wider vocabulary in their native language (which makes it easier to find similarities between languages),
- they have (for a while) a greater working memory capacity,
- they are more likely to have experience of other languages, and of language learning.
For all these reasons, adults can usually learn more words faster than children.
Part of the reason for the belief is that children seem to learn their native language "by magic". While there is certainly something magical about the way they pick up grammar, their learning of new words doesn't come under the same category. In fact, children are quite slow at learning new words, learning on average:
12 - 16 months: 0.3 words/day
16 - 23 months: 0.8 words/day
23 - 30 months: 1.6 words/day
30 months - 6 years: 3.6 words/day
6 - 8 years: 6.6 words/day
8 - 10 years: 12.1 words/day
(from Paul Bloom's (2000) "How Children Learn the Meanings of Words")
Original language can be completely forgotten
The following research is also interesting, since it exposes another cherished myth. A study1 of adults who were born in Korea but adopted by French families in childhood, found not only that they had no conscious memory of Korean, but that imaging showed no difference in brain activation when they heard Korean compared to any other unknown foreign language (activation patterns were different when they heard French).
I don't, however, know the age of the children when they were adopted. It would also be interesting to know whether such children would learn their original language with greater facility - this would imply that present imaging techniques are insufficiently subtle to pick up some differences.