U.S. citizens may also petition for their sons and daughters (unmarried and under the age of 21) as well as their parents, if the U.S. citizen is over the age of 21.
Turning 21 years of age: When an immediate relative child of a U.S. citizen reaches the 21 years of age, he or she generally will become a first preference (F1) category son or daughter (over 21 years of age) of a U.S. citizen, and will no longer have a visa immediately available. This change may result in a significant delay in adjustment of status or visa processing because he or she will now need to wait for an immigrant visa to become available.
Child Status Protection Act: In certain cases, the Child Status Protection Act (CSPA) may allow your child to retain the classification of child even if he/she has reached age 21. Generally, the age will be “frozen” as of the date the U.S. citizen parent files Form I-130.
Getting Married: If an immediate relative child under age 21 gets married, he or she can no longer be classified as an immediate relative and will become a third preference (F3) category – Married son or daughter of a U.S. citizen and a visa would no longer be immediately available.