Hi there jengozum,
If your family group is still showing your teenager as an adult, first ensure that the birthday was changed successfully using these steps:
- Sign in to appleid.apple.com on their Apple ID
- Select Personal Information, then choose Birthday
If it is showing the correct birthday and it still lists them as an adult under Settings > [your name] > Family Sharing, then restart all the devices you use with Family Sharing. This refreshes the software and can resolve unexpected behavior like this.
If this continues after restarting, then go ahead and remove them from the family group and then add them again to force the change to go through. Here's how to do this:
• Leave Family Sharing - Apple Support
• Add a member to a Family Sharing group on iPhone - Apple Support
As far as using parental controls with Family Sharing, this has more information on what you can do: Set up parental controls with Family Sharing on iPhone - Apple Support
Cheers!