But they're not serving ads based on demographics. They're serving ads based on an anonymous ID number that includes no other data about the customer. Why do you think iAds is so unpopular with advertisers??
They can only begin to use effective targeting as they gather data on how effective certain ads were for groups of certain anonymous ID's. They can then compare that to other demographic based data they already have from another source, and "guess" or derive the demographic of the anonymous ID's from that analysis. But all that it worthless if the anonymous ID is reset by the user.
The system is a huge departure from the data mining business model of other ad systems.
Again, why do you think iAd never became a smash hit with advertisers?
Apple can block iAd targeting on users under 13 only to the extent that the device being used is coupled to an AppleID of that minor. If, like my kids, their devices are signed in using the parent's AppleID, then of course Apple doesn't know about them at all.
In light of this, I think Family Sharing is a huge plus, because now I can create AppleID's for my kids, and put the same credit card on them and still share all the purchases, plus it makes it super easy to track what they are purchasing, from a parent's point of view.