I don't think it has anything to do with age. It has everything to do with you adding the phone to your family under settings and declaring that it belongs to a child. You control the definition of child.
I could imagine an abusive partner enabling this to make sure their partner isn’t sexting other people. Given the pushback for AirTags I’m surprised people aren’t more concerned.
Anyone 13 or older can remove themselves from a family sharing group. The only exception is if screen time is enabled and enforced for their device.
Frankly, if you have an abusive partner with physical control over you and a willingness to do this, the fact that Apple supports this technology is the least of your problems.
I’m not sure I’m misunderstanding. This is another feature that allows someone with access to another person’s phone to enable stalkerware like features.