Purchase approval has nothing to do with age ratings of Apps. Those are 2 separate things.
Purchase approvals applies to all apps regardless of ratings. Its exclusively meant as a way to ask permission for downloading and purchasing apps. Even if no age limitation exists, Ask to buy can still be enabled for each and every purchase or download that is attempted.
The age rating simply governs what apps can in fact be attempted to be downloaded or purchased to begin with. If the app cannot be downloaded due to age rating limitations it won't even get as far as Asking for permission to download / purchase it.
If you don't like the way it is set up you can offer Apple feedback about it, but perhaps understanding the difference first would be helpful.
Link-> Product Feedback - Apple