You can either use Restrictions or Guided Access on iPhone. Check out how to set them up on iPhone in this post: How to Set up Parental Controls on iPad
You can restrict the ratings of the app by age not by one particular app by name. Go to settings, general, restrictions, enable restrictions, set up your passcode you want to use, scroll down to allowed content, then you can age restrict music, movies, tv shows, books, apps, and in app purchases.
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You can either use Restrictions or Guided Access on iPhone. Check out how to set them up on iPhone in this post: How to Set up Parental Controls on iPad
You can restrict the ratings of the app by age not by one particular app by name. Go to settings, general, restrictions, enable restrictions, set up your passcode you want to use, scroll down to allowed content, then you can age restrict music, movies, tv shows, books, apps, and in app purchases.