USCIS Policy Memo PM-602-0094 (11/21/2013):  The USCIS memo implements a BIA case, Matter of Wang, which held that derivative beneficiaries in the F-2A category automatically convert to the F-2B category upon turning 21.  When their priority date becomes current, and assuming they are eligible to adjust status, their petitioner does not need to file a separate I-130 petition in their behalf.  The beneficiary can file directly for adjustment of status.  Their priority date will be the one that was obtained when the petitioner filed the I-130 on behalf of the spouse/parent.  Alternatively, the petitioner can file a separate I-130 petition for the son or daughter in the F-2B category and retain the original priority date, assuming the child aged out of derivative status before the principal beneficiary immigrated. For other derivatives who age out in the F-3 or F-4 categories, the agency will not allow for retention of the original priority date when the principal beneficiary immigrates and files a separate petition in the F-2B category for the aged-out derivative child.  The ability of the petitioner in that scenario to retain the original priority date is currently before the U.S. Supreme Court.