Easy, when you buy the device, during first setup, you set up the date of birth of the user. If the user is underage, the parent/guardian sets a password to unl

Easy, when you buy the device, during first setup, you set up the date of birth of the user. If the user is underage, the parent/guardian sets a password to unlock the device if needed before the user is 18. That device then sends "is_below_18" flag to whatever service wants it.