There are plenty of middle ground approaches - one possible one: App default would be restricted time use. To get unrestrxistricted use you use a driver's licen

There are plenty of middle ground approaches - one possible one: App default would be restricted time use. To get unrestrxistricted use you use a driver's license to authenticate birthdate only with OS. Mobile OS only confirming with app that user is above age requirement (does not share birthdate). App can only query if user is above 18yrs and nothing else. Easy peasy.