Laws are often just an acknowledgement of a society's expectation.
"We've all decided that kids under 15 using social isn't great."
The fact that this law exists makes it infinitely b easier for parents to establish and maintain rules in their household, because peer pressure is minimised.
Yes, some kids will still use social before they're 15. Perhaps most kids. However, I think harmfully excessive use will be minimised.
The enforcing part is where this is likely to get shitty. Once they establish this as a law they maybe will try and sue companies that don't provide an age check on their websites. Now if that is possible I am not sure, seeing as many of those are having HQs in Ireland or Netherlands due to tax reasons.
But if that is successful it would mean they actually have to check everyone's age by some means, which means collecting IDs. Which definitely is bad news for users, we all know that data won't be securely stored or deleted.
Not saying you might be right, but this could be solved with a simple API that returns yes/no for the age check, without providing additional information.
Probably networks where users post personal data in conjunction with chat features. Obviously, Wikipedia is not social media in this regard and neither is a mailing list.