When you create a new Apple ID, the purchases from the old Family will remain on their devices/computers, but since the purchases were made when in the old Family, they will not be able to update the purchases.
For the 16 year old, from the article below.
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.
Leave Family Sharing - Apple Support
When changing to the new ID, there may be some data loss, but you can minimize it. If syncing photos with iCloud, ensure Optimization is turned off in Settings/Your Name/iCloud/Photos and/or in Photos/Preferences/iCloud. Then make sure full resolution photos are on the device/computer. When signing out of iCloud, if asked whether you want to retain data, choose yes. Messages conversations will be lost if they are addressed to the ID. See the article below for other places that the old ID should be removed. See Winston Churchill's response in the below discussion. If possible, also set up Family Sharing between the IDs to keep access to your apps/purchases.
How to switch Apple IDs on your iPhone or iPad.
How to transfer all files from one iCloud account to another.
What to do after you change your Apple ID or password - Apple Support.
Family Sharing.
Family Sharing - Set up.
