What is Son/daughter (married/unmarried)?
Son/daughter (married/unmarried) meaning In immigration law, a child becomes a son or daughter when he/she turns 21 or marries. A son or daughter must have once met the definition of a child. Under U.S. immigration law, an unmarried “son or daughter” is a person who was once a “child” but who is now 21 years of age or older. A “married son or daughter” is a person who also satisfied the definition of child, but who is also married, regardless of age.
reference: U.S. Visas – Glossary