Was about to ask the same question.
As the age will be locked on the day the PD is current under FAD chart, my son can continue to be on AOS pending status until GC is received and I don't see a reason for switching to F1.
So, the question is: Is there any law/rule that compels him to switch to a non-immigrant visa such as F1 and not remain on AOS pending status with EAD, once he turns 21?