A green card holder cannot sponsor their parents for US permanent residency. Only citizens above 21 can help their parents get a green card. If you have a green card, you can only sponsor a spouse and unmarried children. You can read more on the official websites.

To petition for your parents (mother or father) to live in the United States as Green Card holders, you must be a U.S. citizen and at least 21 years old. Please see if you meet this requirement. However please note that Green Card holders (permanent residents) may not petition to bring parents to live permanently in the United States.