Typically, a child adopted by a U.S. citizen seeking to be classified as the immediate relative of that citizen for the purpose of procuring an immigrant visa must be legally adopted before the age of 16. In a decision issued last week, the Board of Immigration Appeals determined that it would give effect to adoption decrees entered after a child's 16th birthday, but retroactively dated to a time prior the child's birthday, so long as state law permitted such a retroactive decree and the initial adoption petition was filed in the appropriate state court before the child's 16th birthday.
The full text of Matter of Huang can be found here: http://www.justice.gov/eoir/file/627476/download