Challenging a minor's participation in an Open Source project has likely never happened. It doesn't make sense. For that matter, a lot of legal folk believe strongly that a number of the open source licenses out there are unenforceable anyway. Very likely, it would be possible to legally challenge the contributions of a minor to an open source project and cause hell -- but what would be the point?

Apple's NDA is a completely different story. There most definitely is intellectual property [IP] at risk and that IP very definitely affects Apple's revenue [Darwin as the foundation of OS X]. Furthermore, there are any number of parties in the industry that would love to effectively 'bust' Apple's NDAs for a number of purposes-- even if it is just to cause hell.

Apple has a vested interest in ensuring that their contracts are as tight as possible. If the execution of a contract with a minor through a parent or guardian isn't actually possible or is expensive, they have good reason not to go there [as much as it sucks for Finlay and others in the same situation].