Yes, your U.S. citizen parent can sponsor you for a green card even if you are over 21 years old and married. However, you will fall under the F3 family preference category (Married Sons and Daughters of U.S. Citizens), which has a longer processing time due to visa availability limits. This category allows your spouse and children under 21 to be included in the petition. Since family-sponsored visas are subject to annual caps, wait times can vary significantly, often taking several years. To stay updated on your case, regularly check the Visa Bulletin published by the U.S. Department of State.