Memorandum: Guidance on Processing Petitions for Adopted Alien Children Less than 18 Years of Age Considered a 'Child' under the Immigration and Nationality Act through Public Law 106-139 [open pdf - 74KB]
"This memorandum provides clarification regarding recent action by Congress: Public Law 106-139, signed by the President on December 7, 1999. The law amends the age of an alien 'adopted child' or 'orphan' as defined in the Immigration and Nationality Act (Act) from sixteen years old to eighteen years old in cases where the child's sibling, who is under sixteen, is petitioned by the same U.S. citizen (s). This memorandum updates the December 28, 1999, policy guidance on the subject."
U.S. Citizenship and Immigration Services: http://www.uscis.gov/