Of course you'll always be able to sign up to obscure services such as Lemmy servers. It's always been like that, even with the US law COPPA (nobody really checks).
This law would only be applies to major platforms that have a EU base somewhere (Meta, etc.)
But then the EU is also pushing towards interoperability. If Threads require ID checks and is 15+, but also federates, 13 year olds will just go elsewhere.
I guess the age limit could best be enforced by parents, who would be able to tell their kids they can't use social media before they're old enough. Sure there will still be kids around, but there will be fewer of them.
Get the hell out of here with forced id-d registration. Nobody needs or wants that other than surveillance capitalists.
Fine the parents who let the kids register early, if it gets known. This will only be a problem for those who let their kids be brought up by the internet.