What happens when child turns 13?
What happens to parental controls once a child turns 13?
iPhone 5c, iOS 10
Nothing. Parental Controls remain on regardless of age unless and until they are turned off by whoever set them or knows the Screen Time password for them.
The 13 age limit only applies to being able to remove the account from a Family Sharing group. It does not affect anything else about the account or the restrictions for it.