Child Status Protection Act.

https://www.uscis.gov/green-card/green-card-processes-and-procedures/child-status-protection-act-cspa

CSPA does not change the definition of a child. Instead, CSPA provides a method for calculating a person’s age to see if they meet the definition of a child for immigration purposes. The calculated age is the child’s “CSPA age.” This allows some people to remain classified as children beyond their 21st birthday. However, CSPA does not change the requirement that you must be unmarried in order to remain eligible for classification as a child.