A more thorough explanation:

The Child Status Protection Act (CSPA) is a law that was changed in 2002 to help young people who turned 21 years old before their green card applications were approved by the U.S. Citizenship and Immigration Service (USCIS) and the U.S. Department of State (DOS). Before the CSPA, if a child turned 21 years old, they were no longer able to immigrate or adjust their status with their family. The CSPA solves this problem by "freezing the age" of immediate relative children when their petitioning U.S. citizen parent submits a visa petition on their behalf. The CSPA uses a mathematical formula that allows the amount of time that a visa petition was pending to be subtracted from a child's age.

For example, if a child was 20 years old when their parent submitted a visa petition for them, but the petition took 2 years to be approved, the child's age would be "frozen" at 20 years old, even if they turned 21 during the waiting period. This means that the child would still be eligible to immigrate or adjust their status with their family.

The CSPA was created to address the long delays on the green card application waitlist and the USCIS and DOS average processing times. The law allows young people to retain their classification as a "child" under the Immigration and Nationality Act, even if they have turned 21 years old. This helps families stay together and ensures that children are not penalized for delays that are out of their control.