http://www.ins.usdoj.gov/graphics/howdoi/parents.htm
Who is Eligible to Sponsor a Parent?
If you are a U.S. citizen and at least 21 years old, you are eligible to petition to bring your parents to live and work permanently in the United States.
If you are a lawful permanent resident, you are not eligible to petition to bring your parents to live and work permanently in the United States.