But it is an argument against age restrictions since you could just as easily pass a law that instead required schools to enable various filters. You could even require mainstream devices from major manufacturers to support certain filtering standards. And you could require websites to send self categorization headers.

There is no valid argument for why ID checks are necessary if the goal is simply to get filtering implemented in places such as schools.

If instead the goal is to entirely prohibit all children from using social networks regardless of parental consent then it makes sense. It also makes sense if the goal is actually to violate privacy or something similarly sinister.