It wasn't age locked per se. If you were in Honors English, they assumed you were reading at a higher level and could check out books one grade level higher than you and if you were in on-level English you were not allowed to read above "grade level".
I can understand keeping a 6th grader from checking out a bunch of 1st grade level books, but discouraging kids from pushing themselves was weird
There's nothing that says it can't be done. But I'm sure SCOTUS will have something to say should a case make it to them.
Constitutional amendment would be the best way. Not sure if it would be easier via Congress or State Legislatures given the implosion on Capitol Hill.