Age _indication_ means that when you set up your device or create a user account, you enter a date of birth for the user. The OS then provides a native API to return a user's age bracket (not full date-of-birth). If the user is a minor, the OS will require parental authentication in some way to modify the setting again. This can all be done completely offline. It works because parents almost always buy the devices used by children, and can enter the correct date-of-birth during setup.
Age _verification_ means that some online service has to verify your age, and collects a bunch of (meta)data in the process. This is highly problematic for privacy, security, and the open internet.