Because people share phones with their kids. It's not rare or even mildly unusual. The problem isn't that the app needs to solve this. The problem is the app is useless, along with this whole bizarre "need for age verification" plot that poofed out of existence simultaneously around the whole globe mysteriously a few months ago.
Exactly. "Age verification" is the "think of the children" marketing campaign for "identity verification". Governments don't like anonymity; it makes it harder to find those they consider enemies. But it's hard to market something people don't want and get no benefit from. So, you dress it up in fear and make it easy to villify people who argue against it.
Well, reality called and says: Like ID, drivers license, credit cards and guns: Phones are sth. you dont just "share" with your kids. Also there is an option to guard the ID App with an additional PIN/Biometric.