The role of married children of U.S. citizens in the green card process is determined by their age and the availability of visas in their category.
Under U.S. immigration law, married children of U.S. citizens are classified as "second preference" family-based immigrants, which means they are not considered immediate relatives and therefore may have to wait longer for a green card. Immediate relatives, which include the spouse, parent, and unmarried minor children of U.S. citizens, are not subject to numerical limits and therefore do not have to wait for a visa to become available.
However, the availability of visas in the second preference category is based on a yearly quota and can vary from year to year. If a married child of a U.S. citizen is over 21 years of age, they will be subject to this quota and may have to wait longer for a green card. If they are under 21, they may be eligible for a "derivative" green card as a dependent of their parent, which means they would not be subject to the yearly quota and could potentially receive a green card more quickly.
It is important to note that the green card process for married children of U.S. citizens can be complex and may involve multiple steps, including filing a petition with U.S. Citizenship and Immigration Services (USCIS), attending an interview at a U.S. embassy or consulate, and submitting various forms and documents. Seeking the assistance of an immigration attorney can help ensure that the process goes smoothly and all necessary requirements are met.