USCIS issued a memo offering its officers interim guidance on how to handle petitions which are filed with a request to use an older priority date from a previous petition. In the past two years, the 5th Circuit Court of Appeals issued its decision in Khalid v. Holder and the 9th Circuit Court issued a precedential decision in Cuellar de Osorio v. Holder. These decisions both indicated that children who turned 21 (and thus "Aged Out") before they could become permanent residents with their parents may use the priority dates from their parents' petitions in new petitions filed by their parents to try to obtain permanent residence for these adult sons and daughters. The Cuellar de Osorio case is now before the U.S. Supreme Court and a decision has not yet been made in the case.
The memo is available here.