Can't control tracking feature on my iPhone even after turning 18

I believe you are referring to the “Allow Apps to Request to Track” setting. There are other certain circumstances in which you cannot turn that setting on, take a look below.

• For users with child accounts or who are under age 18 by birth year, signed in with their Apple Account*
• If your Apple Account is managed by an educational or business institution
• If your device is managed and uses a configuration profile that limits tracking
• If your Apple Account was created in the last 3 days

*Also child account age varies by country and region.*

If an app asks to track your activity - Apple Support

Talk to your parents if you can't track your friends on your phone or they can't track you. Your parents have set up controls on your phone.

What might “my tracking feature” mean exactly? Does it mean this?

Locate a device in Find My on iPhone - Apple Support

If not, tell us the precise definition of, as well as the steps you took to turn it on and exactly what you saw when you took these steps.

You are still not providing enough information for us to work with.

Take us step by step through the process.

What happens after each step?