The Child Status Protection Act (CSPA) is a law that was created to help children who are waiting to immigrate to the US with their parents. Before CSPA, if a child turned 21 before they were able to immigrate, they would lose their place in line. CSPA allows children to apply for a green card within one year of when a visa becomes available to them, even if they have already turned 21.
To use CSPA, there are two steps:
Calculate the child's CSPA age on the date that their priority date (the date their visa application was submitted) becomes current. This is done by subtracting the time that the visa petition was waiting for processing from the child's age. In some cases, an additional 45 or 90 days can also be subtracted from the child's age.
If the CSPA age is under 21, the child can apply for a green card within one year of when the priority date becomes current. This rule is a little more complicated in cases where the priority date becomes current and then goes back (retrogresses) within one year.