Nobody seems to be taking issue with the USFSA’s lack of age limits, so why is it so important that ISU must have age limits?
One possible reason:
Skaters competing internationally are representing their country and given a responsibility to deliver results for institutions that are counting on them. That can be considered labor.
Skaters competing nationally are responsible only to themselves. (Of course parents or generous donors who are funding the training may expect results in exchange, but theoretically the parents' primary interest is in the child's well-being rather than immediate return on investment.
Actual sponsorships in which a skater represents a corporation's interests in exchange for financial reimbursement are rare for younger teenagers. I think there have been some for skaters who did well enough at 13-14-15 to bring medals home to the US. And that would be considered labor. Does that include Alysa Liu, whose international opportunities are limited by the ISU's stricter age limits?