Hey there,
You mentioned that the birthday for your Apple ID is already set to over 18. The "Manage activity tracking permissions" section of If an app asks to track your activity has some additional information about this. If your Apple ID was created in the last 3 days, this behavior would be expected.
Also, if you're using an Apple ID managed by educational institute, this would limit your ability to enable this feature. About Managed Apple IDs for education
If the scenarios above don't seem to fit your situation, then we'd recommend checking Settings > General for "Profiles & Device Management." If you're seeing this setting available on your device, that configuration profile is likely limiting tracking on the device.
if that’s doesn’t help let us know !