to file I-824 for her even after her 21?
-Based on the Child Status Protection Act (CSPA), your child CSPA age = your child age when your PD is current (now) - your I-140 pending dates = < 21, so your child is qualified for I-485 filing or your I-824 filing for her, so she has to file I-485 in US or you file I-824 for her within one year form now on even if after she turns 21 as long as the PD is still current.
By the way, she needs to file her I-485 (or you need to file I-824) together with your I-485 filing or after you file your I-485.
See the following links:
