Immigrant minors who have been the victims of neglect, abuse or abandonment can seek classification as a Special Immigrant Juvenile (SIJ). This status may be available to those who are under 21 and unmarried.
Those with SIJ status can apply for an Adjustment of Status if they wish to become legal permanent residents of the U.S. and receive a Green Card. Acceptance is at the discretion of the U.S. Citizenship and Immigration Services (USCIS).
If you currently have SIJ status, you can apply for a Green Card before you reach the age of 21, but there's no age limit for application. Your application won't be denied based on the fact that you've already turned 21 as long as you're still unmarried when the application is filed and adjudicated.
You must be eligible to adjust your status and apply for a Green Card. There are a number of factors that can make someone ineligible. These include convictions for certain criminal offenses, e.g., crimes of "moral turpitude," drug trafficking, money laundering and terrorist activities (including involvement in terrorist organizations).
A person can also be deemed ineligible for health-related reasons, including physical and mental disorders or displayed behavior determined to be a threat to public safety. A person must also present evidence of recent vaccinations for a number of "vaccine-preventable diseases."
If you have SIJ status and wish to seek a Green Card, you should have the guidance of an experienced Maryland immigration attorney. He or she can provide guidance through the process and help improve your chances of becoming a lawful permanent resident of the United States.
Source: U.S. Citizenship and Immigration Services, "Green Card Based on Special Immigrant Juvenile Classification," accessed May 23, 2018