Returning to the thought, one obvious argument for the theater would be that, since they're giving preferential pricing to other people who are also over 40, they are not discriminating against you based on your membership in the protected class. But that would still seem to leave them open to a claim that you should be given the child's pricing as soon as you turn 40.
You might try making an argument that theaters have to give you senior pricing as soon as you turn 40; I don't know what's going on there.