threat to up-and-coming social networks

This is the serious danger. The established guys welcome legislation, especially if it's onerous to comply with but doesn't actually affect the bottom line too much. That essentially locks in the likes of Facebook and Google and ByteDance who can easily afford to comply, while making it harder for smaller social networks to compete.

Maybe the computer platform developers need to introduce some sort of anonymised age/identify verification API

Interesting. If it were a setting locked behind a parental control password, that might make it much harder for kids to lie about, which is an interesting idea. I had previously suggested that, in the vein of the UK's earlier-proposed "go to the pub to get age verified", we could have the government themselves do the age verification, but do it with blinded digital signatures to preserve anonymity of the user from the government—the government would know that you have an age verified token, but not where you used it.

But I quite like this idea of building it in to the platform itself, instead. The user's device would, when parental controls set up the user's age, created a signed token verifying the age using a signing key which is itself signed by three platform vendor. Or, alternatively, they could at that point in time send an API request to the platform vendor to sign their age verification token (blinded), which the device would send (unblinded) along with any account creation requests, or at some other stage when age verification is necessary.