Child accounts can be linked to an adult account (‘parent’/‘guardian’), for users under 13 (varies by country). What matters is the DoB as set when creating the child account. It can’t be changed later.
Create an Apple ID for your child - Apple Support
Child accounts for users over 13 can leave the Family Sharing group, or be removed from the group by the organizer. They can be invited back into the group, but when they are over 18 (varies by country), then child supervision may not be possible anymore. Invitations to a group can be accepted of rejected. No-one can force that.
Users over 13 can create their own account. No inherent child supervision applies then.
Joining a Family Sharing group does not necessarily mean a “restricted” child account. Up to 6 users may form a group, optionally all adults, with one ‘organizer’ that does the invites. The sharing options shares selectable services, not the accounts themselves. No private information is shared, unless you agree to that (e.g. location sharing from Apple devices might be a shared service; it can only be forced for under-13 accounts).
About Family Sharing - Apple
Learn more about what you can do with Family Sharing - Apple Support
Set up Family Sharing - Apple Support
Add a family member to your shared subscriptions - Apple Support
Leave Family Sharing - Apple Support
Use parental controls on your child's iPhone and iPad - Apple Support
When it is just about Apple Music, then all the group users for an Apple Music Family subscription get access to this service on their own account, with separate individual settings and libraries and playlists. Playlists can be shared with any Apple Music subscriber, including outside the Family Sharing group.
Age restrictions apply for some music content (“Explicit”); again based on the DoB for the account. Users can also block “E” rated content for their own account. The organizer does not set this.