Yes, Apple IDs with a birthdate that makes them 13 or older can be removed from a Family Sharing group.
Follow the steps here: link-> Remove a person from Family Sharing - Apple Support
If you are having a specific issue with those steps, then explain exactly what that is.
Check the Apple ID in question, and make sure the birthdate is correctly set. Just because the user of the Apple ID is over 13, does not mean the Apple ID has a birth date that reflects this.
Also of note, is that multiple IDs for a single person should be avoided if at all possible. Its aways best to keep one Apple ID updated and accesible for each person unless a secondary is absolutely necessary for something.
Once the Apple ID is removed from the family sharing group, you may want to request its deletion, so its not still hanging around somewhere unused if its no longer needed. Link-> How to delete your Apple ID account - Apple Support