I foresee this going to Scotus and the only way this can go is that the age of Majority is set at either 18 or 21 for everything.
On the one hand I agree with your assessment.
OTOH - I wonder if the SCOTUS wants to see that issue decided.
(I wonder if I want to see SCOTUS make a ruling on this. Right to vote at age 18? But RKBA at a later age? There will be some convoluted language and logic in that decision, if it gets that far.)