About if citizens or permanent residents has handled immigrant for their minor children, but now their children have been over 18 years of age. And American citizens or permanent residents have handled immigrant for their adult children.
They must fulfill these:
They must go to work or study abroad in USA.
Final result:
When they will have worked in the country accumulative 16 years, and have paid enough taxes (or maybe accumulative ? years)(So they will have paid tax accumulative or over ? years in the country), they can be got the permanent resident status.