I am monitored in family sharing by my wife
i can't sign out of iPhone because I've been set up as child in Family sharing and I'm an adult!
iPhone 11 Pro Max, iOS 14
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see: Leave Family Sharing - Apple Support
Leave your family group
Any family member age 13 or older* can remove themselves from a family group. If you have Screen Time turned on for your account, the family organizer must remove you.
Remove your account from a family group
If your Apple ID is being used to share purchases with a family group that you no longer want to be part of, you can visit your Apple ID account page and remove the account from the family directly. Go to appleid.apple.com and sign in with the Apple ID and password for the account you want to remove. In the Family Sharing section, choose Remove Account > Remove