Re: Screen time : There's no real age restriction in screen time.
If you do not have adequate parental controls in place yet:
You can: Set up parental controls with Family Sharing on iPhone - Apple Support
Eg: says: "You can use Screen Time to manage how much time your children spend in apps and on websites, what content they can access, and who they can communicate with. You can also turn on Ask to Buy, which requires parents to approve eligible purchases and free downloads for their children".
and
See how to: Use parental controls to manage your child's iPhone or iPad - Apple Support
Eg: says:
"When Family Sharing is set up, and your child's Apple Account is added to the Family Sharing group, you need to turn on Content & Privacy Restrictions. Then you're ready to start setting and managing parental controls. Depending on your country or region, and your child's age, some parental controls are on by default.*"
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Else: if your child works at finding ways to bypass the controls you set:
Disobedience in family life is generally up to parental discipline.