Child: 

Unmarried child under the age of 21 years.  A child may be natural born, step or adopted.  If the child is a stepchild, the marriage between the parent and the American citizen must have occurred when the child was under the age of 18.  If the child is adopted, he/she must have been adopted with a full and final adoption when the child was under the age of 16, and the child must have lived with and been legal custody of the parent for at least two years.  An orphan may qualify as a child if he/she has been adopted abroad by an American citizen or if the American citizen parent has filed an immediate-relative (IR) visa petition for him/her to go to the United States for adoption by the American citizen.

In certain visa cases a child continues to be classified as a child after he/she becomes 21, if the petition was filed for him/her when he/she was still under 21 years of age.  For example, an IR-2 child of an American citizen remains a child after the age of 21 if a petition was filed for him/her on or after August 6, 2002, when he/she was still under 21 years old.  The child must meet other requirements of a child as listed above.