International adoption is on the rise, and although the majority of such adoptees are at very early stages of the language acquistion process when they are adopted (below age two), it is not uncommon for children to be adopted at ages where they have quite considerable language skills, even up to age ten or so.
Often, parents make a great effort to help their children preserve their birth language – going on family holidays to countries where it is spoken, encouraging that the child should attend language classes, or trying to find someone to use the language with the child. However, by all accounts, these efforts are usually futile: even the oldest adoptees usually lose their birth language entirely, within months. As adults, they can often not even remember the most basic words of their birth language.
When these children grow up, the question if the language is still there, deeply buried, is often a tantalizing one. Some adoptees try to re-learn it, through taking classes at a language school or university, or through re-immersing themselves in the language. Do they have an advantage over new learners, who have no previous knowledge of this language?
As strange as it sounds, if such advantages exist, they are minimal. If you spoke a language as a child and were then adopted and forgot it, you may become slightly better at pronunciation than your peers with no prior knowledge. For grammar or vocabulary, you will probably have to cram just as hard as they.
There are a few neuroimaging studies trying to find residual knowledge through brain scans, but they, too, show that if such knowledge exists at all, it is very subtle and probably will not be very useful for the purpose of re-learning.