The civil rights act, the ADA only apply to "public accommodations". Many country clubs are not open to the public, they won't provide services to non members and aren't covered by the laws that make discrimination based on age or other classes unlawful.
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.