Apple could (and should) do the same with the app store.
I thought there were already age limits on Apps? When I uploaded mine I set it for 4+ and that's what it stayed ( check out Fridge Magnet Math ).
Seriously, is there a way to set the age limit on an iTunes account? If so, maybe Apple could have made the NIN App rating "17+" or "Explicit" or something.
I would hope they wouldn't leave it up to developers to set the age limits, maybe that will be the change to the App Store approval process? Just rate the App to a higher age limit instead of reject it?