How can I check the age of the app user in advance, e.g. if he orders in a shop?
You can check his user profile to see if he is legally able to make a purchase from you. Alternatively, you can also create a password-protected module and only send the password to all users who are allowed to buy from you. As far as availability in the App Store is concerned, an age limit for the purchase of the app can be specified there.
Last Update: November 29, 2018