Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.
You can’t merge multiple Apple ID accounts.
she continues to use my Apple ID even though she is now a adult.
I want to create a new Apple ID but I wan to keep all my music. how do I export my music to my new Apple ID?
Make a new Apple ID for your child.
Use Family Sharing & Avoid sharing an Apple ID.
See the following Apple articles:
Set up Family Sharing -> Set up Family Sharing - Apple Support
What is Family Sharing? -> What is Family Sharing? - Apple Support
👉 Make sure that each family member has a unique Apple ID
Make sure that each family member has a unique Apple ID - Apple Support
Share App Store, Apple TV, and Apple Books purchases with Family Sharing on iPhone
Share App Store, Apple TV, and Apple Books purchases with Family Sharing on iPhone – Apple Support (UK)
Add a family member to your subscriptions from Apple
Add a family member to your subscriptions from Apple – Apple Support (UK)
Share iCloud+ with your family
Share iCloud+ with your family - Apple Support