A person under a certain age. The upper age limit depends on the particular context and the relevant Act.
The INA provides two different definitions of “child.” One definition of child applies to immigration petitions and lawful permanent resident applications. The other definition of child applies to citizenship and naturalization.
One significant difference between the two definitions of child is that a stepchild is not included in the definition relating to citizenship and naturalization. Although a stepchild may be the stepparent’s “child” for purposes of visa issuance or adjustment of status, the stepchild is not the stepparent’s “child” for purposes of citizenship and naturalization. A stepchild is ineligible for citizenship or naturalization through their U.S. citizen stepparent unless the stepchild is adopted and the adoption meets certain requirements.
Definition for Purposes of Immigration Petitions and Lawful Permanent Residence Applications
Generally, for purposes of immigration petitions and lawful permanent residence (Green Card) applications, a child is an unmarried person under 21 years of age who is:
Definition for Purposes of Citizenship and Naturalization
For the definition of a child for purposes of citizenship and naturalization, please refer to the USCIS Policy Manual, Volume 12, Citizenship and Naturalization, Part H, Children of U.S. Citizens, Chapter 2, Definition of Child and Residence for Citizenship and Naturalization, Section A, Definition of Child [12 USCIS-PM H.2(A)].