This is my understanding of the iOS 12 options a Family Organizer can use to manage what apps a family member under 18  can install:
Disallow install: Screen Time:Content & Privacy Restrictions:iTunes & App Store Purchases
Set age limit: Screen Time:Content & Privacy Restrictions:Content Restrictions and tap on Apps
Ask to buy: Organizer Apple ID:Family Sharing. For each family member under 18 enable “Ask to Buy”.
For now Guides supporting an Explorer’s application use will usually want to enable “Ask to Buy”. It doesn’t help much with managing App installation, but it’s important for managing expenses.
If an age limit works for an Explorer that’s a relatively effective option.
If an Explorer needs at least one adult app, but needs to avoid some of them, you can’t use the age limit method. A Guide has to disallow app installation, which hides App Store.app completely. In this case an Explorer cannot browse the App Store. This is not a great solution.
What we really need is an “Ask to install” in addition to “Ask to buy”, but what we have is Disallow install. I haven’t heard if iOS 13 is better.
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 At this time Guides of special needs adults need to set the adult’s Apple ID birthdate so their calculated age is 13-15 years old.