Age setting for 18+ film

Is this about the web access to Apple TV+ content on the site tv.apple.com?

Then go to your account menu in the top right corner, then select Settings. On the new page, scroll down to Parental Controls, then change the settings there. Those settings will apply to all web access for this site on this account, not just the current device.

The first time you change this, you may be asked to set a passcode for these settings.

On non-Apple devices with this web access, you may be asked to confirm the payment method card, as verification that it is the account holder that makes these changes. Parental controls for Apple TV+ on the web - Apple Support.

If the subscriber is a minor, then this person may not be able to change the settings to beyond his/her own age group, as based on the DoB data for the Apple ID account.