Only a U.S. Citizen who is at least 21 years old is eligible to petition for his/her parents to become lawful permanent residents. Lawful permanent residents are not eligible to petition for his/her parents to become lawful permanent residents.
There is a three-step process for a parent to become a legal immigrant, unless he/she is already in the U.S. in lawful status (see Combined Processing below):
Since a parent is considered an immediate relative and is immediately eligible for an immigrant visa, the USCIS allows for Combined Processing this category. This means that if the parent is legally in the U.S. (through a lawful admission or parole) at the time of filing the Form I-130, Petition for Alien Relative, he/she may also file for adjustment of status at the same time. Combined Processing is only available to beneficiaries/applicants who are in the U.S.