IOS – Is the app age rating a warning

You can try following this procedure (Youtube Video) to create an Apple ID in iTunes without a Credit Card, substituting US for Japan.

You need to follow these steps to create an Apple ID with a U.S. address but without requiring a credit card:

1. Launch iTunes and switch to the iTunes store. Make sure you are not logged in.
2. Scroll to the bottom and change your country to United States.
3. Next go to App store and download any of the free apps.

iTunes will now prompt you to create a new Apple account and this time, you’ll definitely see the ‘none’ option on the Payments screen.

Source.

Your only option is to make a backup of the iOS device on a Mac/PC, then use a third party tool to copy the files into the filesystem as regular files.

From there you can run Disk Inventory X or any similar tool.

Beware that iOS uses compression and doesn't necessarily store all of your files on the local device (photos and music in particular could be in the cloud), so this will not give you an accurate measurement. But it will be as close as you can get.